[IMP] base: Don't show language if only one active

If only one language is active, don't show in the contact
form view (`res.partner`) the language field. Changing the language is
not possible since there is only one language.
Hence the field is useless and therefore hidden.

odoo/addons/base/models/res_partner.py

@@ -154,6 +154,7 @@ class Partner(models.Model):
     ref = fields.Char(string='Reference', index=True)
     lang = fields.Selection(_lang_get, string='Language', default=lambda self: self.env.lang,
                             help="All the emails and documents sent to this contact will be translated in this language.")
+    active_lang_count = fields.Integer(compute='_compute_active_lang_count')
     tz = fields.Selection(_tz_get, string='Timezone', default=lambda self: self._context.get('tz'),
                           help="When printing documents and exporting/importing data, time values are computed according to this timezone.\n"
                                "If the timezone is not set, UTC (Coordinated Universal Time) is used.\n"
@@ -253,6 +254,11 @@ class Partner(models.Model):
         for partner in self:
             partner.display_name = names.get(partner.id)
 
+    def _compute_active_lang_count(self):
+        lang_count = len(self.env['res.lang'].get_installed())
+        for partner in self:
+            partner.active_lang_count = lang_count
+
     @api.depends('tz')
     def _compute_tz_offset(self):
         for partner in self:

odoo/addons/base/views/res_partner_views.xml

@@ -262,8 +262,9 @@
                             <field name="website" widget="url" placeholder="e.g. www.odoo.com"/>
                             <field name="title" options='{"no_open": True}'
                                 attrs="{'invisible': [('is_company', '=', True)]}"/>
-                            <label for="lang"/>
-                            <div class="o_row">
+                            <field name="active_lang_count" invisible="1"/>
+                            <label for="lang" attrs="{'invisible': [('active_lang_count', '&lt;=', 1)]}"/>
+                            <div class="o_row" attrs="{'invisible': [('active_lang_count', '&lt;=', 1)]}">
                                 <field name="lang" placeholder="System (English)"/>
                                 <button
                                     type="action"