diff --git a/addons/account/models/account_journal_dashboard.py b/addons/account/models/account_journal_dashboard.py
index 629ad917748..e0a72234808 100644
--- a/addons/account/models/account_journal_dashboard.py
+++ b/addons/account/models/account_journal_dashboard.py
@@ -6,7 +6,7 @@ from odoo import models, api, _, fields
 from odoo.osv import expression
 from odoo.release import version
 from odoo.tools import DEFAULT_SERVER_DATE_FORMAT as DF, safe_eval
-from odoo.tools.misc import formatLang, format_date as odoo_format_date
+from odoo.tools.misc import formatLang, format_date as odoo_format_date, get_lang
 import random
 
 import ast
@@ -90,7 +90,7 @@ class account_journal(models.Model):
         data = []
         today = datetime.today()
         last_month = today + timedelta(days=-30)
-        locale = self._context.get('lang') or 'en_US'
+        locale = get_lang(self.env).code
 
         #starting point of the graph is the last statement
         last_stmt = BankStatement.search([('journal_id', '=', self.id), ('date', '<=', today.strftime(DF))], order='date desc, id desc', limit=1)
@@ -139,7 +139,7 @@ class account_journal(models.Model):
         data = []
         today = fields.Datetime.now(self)
         data.append({'label': _('Due'), 'value':0.0, 'type': 'past'})
-        day_of_week = int(format_datetime(today, 'e', locale=self._context.get('lang') or 'en_US'))
+        day_of_week = int(format_datetime(today, 'e', locale=get_lang(self.env).code))
         first_day_of_week = today + timedelta(days=-day_of_week+1)
         for i in range(-1,4):
             if i==0:
@@ -150,9 +150,9 @@ class account_journal(models.Model):
                 start_week = first_day_of_week + timedelta(days=i*7)
                 end_week = start_week + timedelta(days=6)
                 if start_week.month == end_week.month:
-                    label = str(start_week.day) + '-' +str(end_week.day)+ ' ' + format_date(end_week, 'MMM', locale=self._context.get('lang') or 'en_US')
+                    label = str(start_week.day) + '-' + str(end_week.day) + ' ' + format_date(end_week, 'MMM', locale=get_lang(self.env).code)
                 else:
-                    label = format_date(start_week, 'd MMM', locale=self._context.get('lang') or 'en_US')+'-'+format_date(end_week, 'd MMM', locale=self._context.get('lang') or 'en_US')
+                    label = format_date(start_week, 'd MMM', locale=get_lang(self.env).code) + '-' + format_date(end_week, 'd MMM', locale=get_lang(self.env).code)
             data.append({'label':label,'value':0.0, 'type': 'past' if i<0 else 'future'})
 
         # Build SQL query to find amount aggregated by week
diff --git a/addons/account/wizard/account_invoice_send.py b/addons/account/wizard/account_invoice_send.py
index 2b98c37e9a2..99d745ff94b 100644
--- a/addons/account/wizard/account_invoice_send.py
+++ b/addons/account/wizard/account_invoice_send.py
@@ -4,6 +4,7 @@
 from odoo import api, fields, models, _
 from odoo.addons.mail.wizard.mail_compose_message import _reopen
 from odoo.exceptions import UserError
+from odoo.tools.misc import get_lang
 
 
 class AccountInvoiceSend(models.TransientModel):
@@ -103,7 +104,7 @@ class AccountInvoiceSend(models.TransientModel):
             active_ids = self.env.context.get('active_ids', self.res_id)
             active_records = self.env[self.model].browse(active_ids)
             langs = active_records.mapped('partner_id.lang')
-            default_lang = self.env.context.get('lang', 'en_US')
+            default_lang = get_lang(self.env)
             for lang in (set(langs) or [default_lang]):
                 active_ids_lang = active_records.filtered(lambda r: r.partner_id.lang == lang).ids
                 self_lang = self.with_context(active_ids=active_ids_lang, lang=lang)
diff --git a/addons/account/wizard/account_report_common.py b/addons/account/wizard/account_report_common.py
index 3d113ff58bf..e6317daea8c 100644
--- a/addons/account/wizard/account_report_common.py
+++ b/addons/account/wizard/account_report_common.py
@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 
 from odoo import api, fields, models, _
+from odoo.tools.misc import get_lang
 
 
 class AccountCommonReport(models.TransientModel):
@@ -43,5 +44,5 @@ class AccountCommonReport(models.TransientModel):
         data['model'] = self.env.context.get('active_model', 'ir.ui.menu')
         data['form'] = self.read(['date_from', 'date_to', 'journal_ids', 'target_move', 'company_id'])[0]
         used_context = self._build_contexts(data)
-        data['form']['used_context'] = dict(used_context, lang=self.env.context.get('lang') or 'en_US')
+        data['form']['used_context'] = dict(used_context, lang=get_lang(self.env).code)
         return self.with_context(discard_logo_check=True)._print_report(data)
diff --git a/addons/base_gengo/wizard/base_gengo_translations.py b/addons/base_gengo/wizard/base_gengo_translations.py
index 24a9d060bdd..898ef0a0656 100644
--- a/addons/base_gengo/wizard/base_gengo_translations.py
+++ b/addons/base_gengo/wizard/base_gengo_translations.py
@@ -8,6 +8,7 @@ import uuid
 
 from odoo import api, fields, models, tools, _
 from odoo.exceptions import UserError, ValidationError
+from odoo.tools.misc import get_lang
 
 _logger = logging.getLogger(__name__)
 
@@ -31,9 +32,7 @@ class BaseGengoTranslations(models.TransientModel):
         res = super(BaseGengoTranslations, self).default_get(fields)
         res['authorized_credentials'], gengo = self.gengo_authentication()
         if 'lang_id' in fields:
-            res['lang_id'] = self.env['res.lang']._lang_get_id(
-                self.env.context.get('lang') or 'en_US'
-            )
+            res['lang_id'] = get_lang(self.env).id
         return res
 
     sync_type = fields.Selection([
diff --git a/addons/calendar/controllers/main.py b/addons/calendar/controllers/main.py
index 69bd2b9ade7..64b8812c7c2 100644
--- a/addons/calendar/controllers/main.py
+++ b/addons/calendar/controllers/main.py
@@ -9,6 +9,7 @@ import odoo.http as http
 from odoo.http import request
 from odoo import SUPERUSER_ID
 from odoo import registry as registry_get
+from odoo.tools.misc import get_lang
 
 
 class CalendarController(http.Controller):
@@ -43,7 +44,7 @@ class CalendarController(http.Controller):
             if not attendee:
                 return request.not_found()
             timezone = attendee.partner_id.tz
-            lang = attendee.partner_id.lang or 'en_US'
+            lang = attendee.partner_id.lang or get_lang(request.env).code
             event = env['calendar.event'].with_context(tz=timezone, lang=lang).browse(int(id))
 
             # If user is internal and logged, redirect to form view of event
diff --git a/addons/calendar/models/calendar.py b/addons/calendar/models/calendar.py
index 74073286b36..12f64409f2d 100644
--- a/addons/calendar/models/calendar.py
+++ b/addons/calendar/models/calendar.py
@@ -20,6 +20,7 @@ from odoo import tools
 from odoo.addons.base.models.res_partner import _tz_get
 from odoo.osv import expression
 from odoo.tools.translate import _
+from odoo.tools.misc import get_lang
 from odoo.tools import DEFAULT_SERVER_DATE_FORMAT, DEFAULT_SERVER_DATETIME_FORMAT, pycompat
 from odoo.exceptions import UserError, ValidationError
 
@@ -1352,11 +1353,11 @@ class Meeting(models.Model):
 
         elif interval == 'month':
             # Localized month name and year
-            result = babel.dates.format_date(date=date, format='MMMM y', locale=self._context.get('lang') or 'en_US')
+            result = babel.dates.format_date(date=date, format='MMMM y', locale=get_lang(self.env).code)
 
         elif interval == 'dayname':
             # Localized day name
-            result = babel.dates.format_date(date=date, format='EEEE', locale=self._context.get('lang') or 'en_US')
+            result = babel.dates.format_date(date=date, format='EEEE', locale=get_lang(self.env).code)
 
         elif interval == 'time':
             # Localized time
diff --git a/addons/product/models/product_pricelist.py b/addons/product/models/product_pricelist.py
index 25552049c02..754a456d7d1 100644
--- a/addons/product/models/product_pricelist.py
+++ b/addons/product/models/product_pricelist.py
@@ -5,6 +5,7 @@ from itertools import chain
 
 from odoo import api, fields, models, tools, _
 from odoo.exceptions import UserError, ValidationError
+from odoo.tools.misc import get_lang
 
 
 class Pricelist(models.Model):
@@ -40,7 +41,7 @@ class Pricelist(models.Model):
         if name and operator == '=' and not args:
             # search on the name of the pricelist and its currency, opposite of name_get(),
             # Used by the magic context filter in the product search view.
-            query_args = {'name': name, 'limit': limit, 'lang': self._context.get('lang') or 'en_US'}
+            query_args = {'name': name, 'limit': limit, 'lang': get_lang(self.env).code}
             query = """SELECT p.id
                        FROM ((
                                 SELECT pr.id, pr.name
diff --git a/addons/rating/controllers/main.py b/addons/rating/controllers/main.py
index ac585f7de56..7cbeeb85c72 100644
--- a/addons/rating/controllers/main.py
+++ b/addons/rating/controllers/main.py
@@ -6,6 +6,7 @@ import werkzeug
 from odoo import http
 from odoo.http import request
 from odoo.tools.translate import _
+from odoo.tools.misc import get_lang
 
 
 class Rating(http.Controller):
@@ -22,7 +23,7 @@ class Rating(http.Controller):
             10: _("satisfied")
         }
         rating.write({'rating': rate, 'consumed': True})
-        lang = rating.partner_id.lang or 'en_US'
+        lang = rating.partner_id.lang or get_lang(request.env).code
         return request.env['ir.ui.view'].with_context(lang=lang).render_template('rating.rating_external_page_submit', {
             'rating': rating, 'token': token,
             'rate_name': rate_names[rate], 'rate': rate
@@ -35,7 +36,7 @@ class Rating(http.Controller):
             return request.not_found()
         record_sudo = request.env[rating.res_model].sudo().browse(rating.res_id)
         record_sudo.rating_apply(rate, token=token, feedback=kwargs.get('feedback'))
-        lang = rating.partner_id.lang or 'en_US'
+        lang = rating.partner_id.lang or get_lang(request.env).code
         return request.env['ir.ui.view'].with_context(lang=lang).render_template('rating.rating_external_page_view', {
             'web_base_url': request.env['ir.config_parameter'].sudo().get_param('web.base.url'),
             'rating': rating,
diff --git a/addons/sale_timesheet/models/project_overview.py b/addons/sale_timesheet/models/project_overview.py
index afa3e5cc642..1230ce8c6fa 100644
--- a/addons/sale_timesheet/models/project_overview.py
+++ b/addons/sale_timesheet/models/project_overview.py
@@ -6,6 +6,7 @@ import json
 
 from odoo import fields, _, models
 from odoo.tools import float_round
+from odoo.tools.misc import get_lang
 
 from odoo.addons.web.controllers.main import clean_action
 
@@ -256,7 +257,7 @@ class Project(models.Model):
 
         def _to_short_month_name(date):
             month_index = fields.Date.from_string(date).month
-            return babel.dates.get_month_names('abbreviated', locale=self.env.context.get('lang', 'en_US'))[month_index]
+            return babel.dates.get_month_names('abbreviated', locale=get_lang(self.env).code)[month_index]
 
         header_names = [_('Name'), _('Before')] + [_to_short_month_name(date) for date in ts_months] + [_('Total'), _('Sold'), _('Remaining')]
 
diff --git a/addons/web/models/models.py b/addons/web/models/models.py
index e85de4f6c45..3ef871cabe0 100644
--- a/addons/web/models/models.py
+++ b/addons/web/models/models.py
@@ -5,6 +5,7 @@ import pytz
 from odoo import _, api, fields, models
 from odoo.osv.expression import AND
 from odoo.tools import lazy
+from odoo.tools.misc import get_lang
 from odoo.exceptions import UserError
 
 
@@ -142,7 +143,7 @@ class Base(models.AbstractModel):
 
             # Again, imitating what _read_group_format_result and _read_group_prepare_data do
             if group_by_value and field_type in ['date', 'datetime']:
-                locale = self._context.get('lang') or 'en_US'
+                locale = get_lang(self.env).code
                 group_by_value = fields.Datetime.to_datetime(group_by_value)
                 group_by_value = pytz.timezone('UTC').localize(group_by_value)
                 tz_info = None
diff --git a/addons/website_blog/controllers/main.py b/addons/website_blog/controllers/main.py
index 42a7b946506..0d691bc427c 100644
--- a/addons/website_blog/controllers/main.py
+++ b/addons/website_blog/controllers/main.py
@@ -13,6 +13,7 @@ from odoo.addons.http_routing.models.ir_http import slug, unslug
 from odoo.addons.website.controllers.main import QueryURL
 from odoo.http import request
 from odoo.tools import html2plaintext
+from odoo.tools.misc import get_lang
 
 
 class WebsiteBlog(http.Controller):
@@ -43,7 +44,7 @@ class WebsiteBlog(http.Controller):
             group['date_begin'] = start
             group['date_end'] = end
 
-            locale = request.context.get('lang') or 'en_US'
+            locale = get_lang(request.env).code
             start = pytz.UTC.localize(fields.Datetime.from_string(start))
             tzinfo = pytz.timezone(request.context.get('tz', 'utc') or 'utc')
 
diff --git a/addons/website_event/controllers/main.py b/addons/website_event/controllers/main.py
index 1476c02da8a..20031a67781 100644
--- a/addons/website_event/controllers/main.py
+++ b/addons/website_event/controllers/main.py
@@ -12,6 +12,7 @@ from odoo import fields, http, _
 from odoo.addons.http_routing.models.ir_http import slug
 from odoo.addons.website.controllers.main import QueryURL
 from odoo.http import request
+from odoo.tools.misc import get_lang
 
 
 class WebsiteEventController(http.Controller):
@@ -229,7 +230,7 @@ class WebsiteEventController(http.Controller):
     def get_formated_date(self, event):
         start_date = fields.Datetime.from_string(event.date_begin).date()
         end_date = fields.Datetime.from_string(event.date_end).date()
-        month = babel.dates.get_month_names('abbreviated', locale=event.env.context.get('lang') or 'en_US')[start_date.month]
+        month = babel.dates.get_month_names('abbreviated', locale=get_lang(event.env).code)[start_date.month]
         return ('%s %s%s') % (month, start_date.strftime("%e"), (end_date != start_date and ("-" + end_date.strftime("%e")) or ""))
 
     @http.route('/event/get_country_event_list', type='json', auth='public', website=True)
diff --git a/addons/website_sale/controllers/backend.py b/addons/website_sale/controllers/backend.py
index 6c72971749d..f4f7f8c1210 100644
--- a/addons/website_sale/controllers/backend.py
+++ b/addons/website_sale/controllers/backend.py
@@ -7,6 +7,7 @@ from datetime import datetime, timedelta, time
 from odoo import fields, http, _
 from odoo.addons.website.controllers.backend import WebsiteBackend
 from odoo.http import request
+from odoo.tools.misc import get_lang
 
 
 class WebsiteSaleBackend(WebsiteBackend):
@@ -175,7 +176,7 @@ class WebsiteSaleBackend(WebsiteBackend):
         sales_graph = [{
             '0': fields.Date.to_string(d) if not previous else fields.Date.to_string(d + timedelta(days=days_between)),
             # Respect read_group format in models.py
-            '1': daily_sales_dict.get(babel.dates.format_date(d, format='dd MMM yyyy', locale=request.env.context.get('lang') or 'en_US'), 0)
+            '1': daily_sales_dict.get(babel.dates.format_date(d, format='dd MMM yyyy', locale=get_lang(request.env).code), 0)
         } for d in date_list]
 
         return sales_graph
diff --git a/odoo/addons/base/models/ir_qweb.py b/odoo/addons/base/models/ir_qweb.py
index 0ddb06744e0..07fe2d1592f 100644
--- a/odoo/addons/base/models/ir_qweb.py
+++ b/odoo/addons/base/models/ir_qweb.py
@@ -13,6 +13,7 @@ from werkzeug import urls
 
 from odoo import api, models, tools
 from odoo.tools.safe_eval import assert_valid_codeobj, _BUILTINS, _SAFE_OPCODES
+from odoo.tools.misc import get_lang
 from odoo.http import request
 from odoo.modules.module import get_resource_path
 
@@ -112,7 +113,7 @@ class IrQWeb(models.AbstractModel, QWeb):
         return super(IrQWeb, self).compile(id_or_xml_id, options=options)
 
     def load(self, name, options):
-        lang = options.get('lang', 'en_US')
+        lang = options.get('lang', get_lang(self.env).code)
         env = self.env
         if lang != env.context.get('lang'):
             env = env(context=dict(env.context, lang=lang))
@@ -148,7 +149,7 @@ class IrQWeb(models.AbstractModel, QWeb):
     # compile directives
 
     def _compile_directive_lang(self, el, options):
-        lang = el.attrib.pop('t-lang', 'en_US')
+        lang = el.attrib.pop('t-lang', get_lang(self.env).code)
         if el.get('t-call-options'):
             el.set('t-call-options', el.get('t-call-options')[0:-1] + u', "lang": %s}' % lang)
         else:
diff --git a/odoo/addons/base/models/ir_qweb_fields.py b/odoo/addons/base/models/ir_qweb_fields.py
index c275707377b..329d9b3c55b 100644
--- a/odoo/addons/base/models/ir_qweb_fields.py
+++ b/odoo/addons/base/models/ir_qweb_fields.py
@@ -10,6 +10,7 @@ from lxml import etree
 import math
 
 from odoo.tools import html_escape as escape, posix_to_ldml, safe_eval, float_utils, format_date, format_duration, pycompat
+from odoo.tools.misc import get_lang
 
 import logging
 _logger = logging.getLogger(__name__)
@@ -130,13 +131,11 @@ class FieldConverter(models.AbstractModel):
         """ user_lang()
 
         Fetches the res.lang record corresponding to the language code stored
-        in the user's context. Fallbacks to en_US if no lang is present in the
-        context *or the language code is not valid*.
+        in the user's context.
 
         :returns: Model[res.lang]
         """
-        lang_code = self._context.get('lang') or 'en_US'
-        return self.env['res.lang']._lang_get(lang_code)
+        return get_lang(self.env)
 
 
 class IntegerConverter(models.AbstractModel):
diff --git a/odoo/models.py b/odoo/models.py
index 0a076effedd..479b86bbf01 100644
--- a/odoo/models.py
+++ b/odoo/models.py
@@ -56,7 +56,7 @@ from .tools import frozendict, lazy_classproperty, lazy_property, ormcache, \
                    groupby
 from .tools.config import config
 from .tools.func import frame_codeinfo
-from .tools.misc import CountingStream, clean_context, DEFAULT_SERVER_DATETIME_FORMAT, DEFAULT_SERVER_DATE_FORMAT
+from .tools.misc import CountingStream, clean_context, DEFAULT_SERVER_DATETIME_FORMAT, DEFAULT_SERVER_DATE_FORMAT, get_lang
 from .tools.safe_eval import safe_eval
 from .tools.translate import _
 from .tools import date_utils
@@ -1991,7 +1991,7 @@ class BaseModel(MetaModel('DummyModel', (object,), {'_register': False})):
                 if ftype == 'many2one':
                     value = value[0]
                 elif ftype in ('date', 'datetime'):
-                    locale = self._context.get('lang') or 'en_US'
+                    locale = get_lang(self.env).code
                     fmt = DEFAULT_SERVER_DATETIME_FORMAT if ftype == 'datetime' else DEFAULT_SERVER_DATE_FORMAT
                     tzinfo = None
                     range_start = value
diff --git a/odoo/osv/expression.py b/odoo/osv/expression.py
index e15ea963abd..38d503af0f9 100644
--- a/odoo/osv/expression.py
+++ b/odoo/osv/expression.py
@@ -123,6 +123,7 @@ from zlib import crc32
 from datetime import date, datetime, time
 import odoo.modules
 from odoo.tools import pycompat
+from odoo.tools.misc import get_lang
 from ..models import MAGIC_COLUMNS, BaseModel
 import odoo.tools as tools
 
@@ -1145,7 +1146,7 @@ class expression(object):
 
                     params = (
                         model._name + ',' + left,
-                        model.env.lang or 'en_US',
+                        get_lang(model.env).code,
                         'model',
                         right,
                     )
diff --git a/odoo/tools/misc.py b/odoo/tools/misc.py
index a9268db1eed..b65cd23cf6f 100644
--- a/odoo/tools/misc.py
+++ b/odoo/tools/misc.py
@@ -1162,6 +1162,21 @@ else:
     def html_escape(text):
         return werkzeug.utils.escape(text)
 
+
+def get_lang(env, lang_code=False):
+    """
+    Retrieve the first lang object installed, by checking the parameter lang_code,
+    the context and then the company. If no lang is installed from those variables,
+    fallback on the first lang installed in the system.
+    :param str lang_code: the locale (i.e. en_US)
+    :return res.lang: the first lang found that is installed on the system.
+    """
+    langs = [code for code, _ in env['res.lang'].get_installed()]
+    for code in [lang_code, env.context.get('lang'), env.user.company_id.partner_id.lang, langs[0]]:
+        if code in langs:
+            return env['res.lang']._lang_get(code)
+
+
 def formatLang(env, value, digits=None, grouping=True, monetary=False, dp=False, currency_obj=False):
     """
         Assuming 'Account' decimal.precision=3:
@@ -1182,9 +1197,7 @@ def formatLang(env, value, digits=None, grouping=True, monetary=False, dp=False,
     if isinstance(value, str) and not value:
         return ''
 
-    langs = [code for code, _ in env['res.lang'].get_installed()]
-    lang_code = env.context['lang'] if env.context.get('lang') in langs else (env.user.company_id.partner_id.lang or langs[0])
-    lang_obj = env['res.lang']._lang_get(lang_code)
+    lang_obj = get_lang(env)
 
     res = lang_obj.format('%.' + str(digits) + 'f', value, grouping=grouping, monetary=monetary)
 
@@ -1221,7 +1234,7 @@ def format_date(env, value, lang_code=False, date_format=False):
         else:
             value = odoo.fields.Datetime.from_string(value)
 
-    lang = env['res.lang']._lang_get(lang_code or env.context.get('lang') or 'en_US')
+    lang = get_lang(env, lang_code)
     locale = babel.Locale.parse(lang.code)
     if not date_format:
         date_format = posix_to_ldml(lang.date_format, locale=locale)
@@ -1240,7 +1253,7 @@ def parse_date(env, value, lang_code=False):
         :return: date object from the localized string
         :rtype: datetime.date
     '''
-    lang = env['res.lang']._lang_get(lang_code or env.context.get('lang') or 'en_US')
+    lang = get_lang(env, lang_code)
     locale = babel.Locale.parse(lang.code)
     try:
         return babel.dates.parse_date(value, locale=locale)
@@ -1271,8 +1284,7 @@ def format_datetime(env, value, tz=False, dt_format='medium', lang_code=False):
     except Exception:
         localized_datetime = utc_datetime
 
-    lang_code = lang_code or env.context.get('lang') or 'en_US'
-    lang = env['res.lang']._lang_get(lang_code)
+    lang = get_lang(env, lang_code)
 
     locale = babel.Locale.parse(lang.code or lang_code)  # lang can be inactive, so `lang`is empty
     if not dt_format:
@@ -1302,7 +1314,7 @@ def format_time(env, value, tz=False, time_format='medium', lang_code=False):
     if not value:
         return ''
 
-    lang = env['res.lang']._lang_get(lang_code or env.context.get('lang') or 'en_US')
+    lang = get_lang(env, lang_code)
     locale = babel.Locale.parse(lang.code)
     if not time_format:
         time_format = posix_to_ldml(lang.time_format, locale=locale)
@@ -1320,7 +1332,7 @@ def _format_time_ago(env, time_delta, lang_code=False, add_direction=True):
 
 def format_amount(env, amount, currency, lang_code=False):
     fmt = "%.{0}f".format(currency.decimal_places)
-    lang = env['res.lang']._lang_get(lang_code or env.context.get('lang') or 'en_US')
+    lang = get_lang(env, lang_code)
 
     formatted_amount = lang.format(fmt, currency.round(amount), grouping=True, monetary=True)\
         .replace(r' ', u'\N{NO-BREAK SPACE}').replace(r'-', u'-\N{ZERO WIDTH NO-BREAK SPACE}')