[IMP] base: no fallback on _lang_get_id

It was very misleading that
    self.env['res.lang']._lang_get('fr_FR')
used to return any language if fr_FR was not active or did not exited

There is a very frequent need to get a language from the code
There is rarely a need to get any language

odoo/addons/base/models/res_lang.py

@@ -179,13 +179,10 @@ class Lang(models.Model):
 
     @tools.ormcache('code')
     def _lang_get_id(self, code):
-        return (self.search([('code', '=', code)]) or
-                self.search([('code', '=', 'en_US')]) or
-                self.search([], limit=1)).id
+        return self.with_context(active_test=True).search([('code', '=', code)]).id
 
-    @api.model
-    @api.returns('self', lambda value: value.id)
     def _lang_get(self, code):
+        """ Return the language using this code if it is active """
         return self.browse(self._lang_get_id(code))
 
     @tools.ormcache('self.code', 'monetary')