[FIX] web_editor: prevent en_US key to be missing from arch_db

Previous commit is fixing the code so that a view without `en_US` in
arch_db jsonb value won't make the code crash (`get_related_views`).

The commit before that actually improved the custom snippets so the
translations are forwarded to it when it's saved.
But that new code was introducing a way to have an arch without `en_US`
value, which led to the bug described above.

While the previous commit fixed the consequence of the issue, this
commit solves the root cause of the issue, introduced 2 commits before.

Arguably, this commit could be squashed in the one 2 commits ago, but
it makes things more clear to figure.

task-3375518

closes odoo/odoo#150945

X-original-commit: bf598d9098b4be36542a9a91c0dd1055cf36aed5
Signed-off-by: Romain Derie (rde) <rde@odoo.com>

addons/web_editor/models/ir_ui_view.py

@@ -150,6 +150,11 @@ class IrUiView(models.Model):
         existing_translation_dictionary.update(extra_translation_dictionary)
         translation_dictionary = existing_translation_dictionary
 
+        # The `en_US` jsonb value should always be set, even if english is not
+        # installed. If we don't do this, the custom snippet `arch_db` will only
+        # have a `fr_BE` key but no `en_US` key.
+        langs.add('en_US')
+
         # 2. Set translations
         new_value = {
             lang: field_to.translate(lambda term: translation_dictionary.get(term, {}).get(lang), record_to[name_field_to])