untranslated placeholders: ensure they plugins always have the same language!

[Ben Stähli]

otherwise we'll face tree errors when sorting.

• use a pre_save signal for cms_cmsplugin creation?
• OR: rethink untranslated placeholders, as fallbacks, that are always shown (less complex monkey patching!!!).
• ordering plugins when in other than the plugin's language is generally a problem, as the placehoderadminmixin does some checks - another monkey patch, for move_plugin (adapt placeholder_id, and call the original...) https://github.com/divio/django-cms/blob/release/3.4.x/cms/admin/placeholderadmin.py#L667

[Ben Stähli]

editmode_fallback is preferred, so editors can still override some placeholders...no action for now