[FIX] core: update model terms for en_US and other

For model_terms translated fields if a translation for a lang(fr_FR) has never
been defined

before this commit
when update translations for both en_US and fr_FR, the new translation for fr_FR
cannot be saved.

after this commit
new translations can be correctly saved when en_US and fr_FR are updated at the
same time.

closes odoo/odoo#135103

X-original-commit: ef4b195eeac178a14eb47f4e6cc61ddc97f78866
Signed-off-by: Raphael Collet <rco@odoo.com>
Signed-off-by: Chong Wang (cwg) <cwg@odoo.com>

odoo/addons/base/tests/test_translate.py

@@ -1139,6 +1139,16 @@ class TestXMLTranslation(TransactionCase):
         self.assertEqual(view.with_context(lang='fr_FR').arch_db, '<form string="X">Pain et fromage<div>Fourchette2</div></form>')
         self.assertEqual(view.with_context(lang='nl_NL').arch_db, view_nl)
 
+        # update translations for fallback values and en_US
+        self.env['res.lang']._activate_lang('es_ES')
+        self.assertEqual(view.with_context(lang='es_ES').arch_db, '<form string="X">Bread and cheese<div>Fork2</div></form>')
+        view.update_field_translations('arch_db', {
+            'en_US': {'Fork2': 'Fork3'},
+            'es_ES': {'Fork2': 'Tenedor3'}
+        })
+        self.assertEqual(view.with_context(lang='en_US').arch_db, '<form string="X">Bread and cheese<div>Fork3</div></form>')
+        self.assertEqual(view.with_context(lang='es_ES').arch_db, '<form string="X">Bread and cheese<div>Tenedor3</div></form>')
+
 
 class TestHTMLTranslation(TransactionCase):
     def test_write_non_existing(self):

odoo/models.py

@@ -3558,8 +3558,9 @@ class BaseModel(metaclass=MetaModel):
             if not old_translations:
                 return False
             new_translations = old_translations
+            old_value_en = old_translations.get('en_US')
             for lang, translation in translations.items():
-                old_value = new_translations.get(lang) or new_translations.get('en_US')
+                old_value = new_translations.get(lang, old_value_en)
                 if digest:
                     old_terms = field.get_trans_terms(old_value)
                     old_terms_digested2value = {digest(old_term): old_term for old_term in old_terms}