Better langcode mapping to TinyMCE translation files

[indigoxela]

While testing for latest library update, I realized, that there should be some mapping for Backdrop langcodes to TinyMCE lang pack files.
Otherwise many translations won't load automatically.

It's always possible to override that by altering tiny_options, either by hook_tinymce_options_alter or the profile json file. But most people won't know.

Disadvantage: as Tiny's lang pack files are pretty volatile, this needs to get checked and very likely updated with every library (lang pack) update.

[indigoxela]

The nice thing, though - as now we check for the existence of the translation js file, no more "file not found" nagging in the console, when tiny tries to fetch a file that's not available. 😉

More testing later.