[IMP] base: avoid empty language confusion

In User preferences, the empty language was confusing, rename it to 'System
(English)' instead of blank.
Rename English language as English (US).

Task ID : 1921583

closes odoo/odoo#30096

odoo/addons/base/data/res.lang.csv

@@ -1,5 +1,5 @@
 "id","name","code","iso_code","direction","grouping","decimal_point","thousands_sep","date_format","time_format","translatable","week_start"
-"base.lang_en","English","en_US","en","Left-to-Right","[3,0]",".",",","%m/%d/%Y","%H:%M:%S","True","7"
+"base.lang_en","English (US)","en_US","en","Left-to-Right","[3,0]",".",",","%m/%d/%Y","%H:%M:%S","True","7"
 "base.lang_am_ET","Amharic / አምሃርኛ","am_ET","am_ET","Left-to-Right","[3,0]",".",",","%d/%m/%Y","%I:%M:%S","True","7"
 "base.lang_ar_SY","Arabic / الْعَرَبيّة","ar_SY","ar","Right-to-Left","[3,0]",".",",","%d %b, %Y","%I:%M:%S","True","6"
 "base.lang_eu_ES","Basque / Euskara","eu_ES","eu_ES","Left-to-Right","[]",",",,"%a, %Y.eko %bren %da","%H:%M:%S","True","1"

odoo/addons/base/views/res_users_views.xml

@@ -342,7 +342,7 @@
                     </h1>
                     <button name="preference_change_password" type="object" string="Change password" class="oe_link"/>
                     <group name="preferences" col="4">
-                        <field name="lang" readonly="0"/>
+                        <field name="lang" readonly="0" placeholder="System (English)"/>
                         <field name="tz" widget="timezone_mismatch" options="{'tz_offset_field': 'tz_offset'}" readonly="0"/>
                         <field name="tz_offset" invisible="1"/>
                         <field name="company_id" options="{'no_create': True}" readonly="0"