`pat-tinymce`: Provide a proper fallback when a language is not found

[frapell]

This is coming from plone/plone.app.mosaic#511

When the site is set to a language for which there is no specific translation (Example with en_US), then tinymce is not properly initialized.
I haven't found yet why this seems to be a problem when there are multiple instances of tinymce in "inline mode" (as described in the original ticket in mosaic), and it doesn't seem to cause any apparent issues for when tinymce is not used in "inline mode" (Like the default case for the "text" field in a normal add/edit form), other than a 404 when trying to get the language .JS file.

In any case, it should be properly fixed, as a suggestion I believe it first should try to find the proper language with the variant, if not found, try the language without variant, and finally fallback to "en".