CKEditor translation not loaded for german formal (de-DE) / espanol ecuador (es-EC)

[kesselb]

Steps to reproduce

We have german informal (de) and german formal (de-DE). The translation only works when using the informal version.

1. Set the language to german formal (de-DE)
2. Compose a new message
3. Ensure formatting is enabled
4. See the strings are not translated

Expected behavior

The strings should be german

Actual behavior

The strings are english

Mail app version

main
stable3.7

Additional info

[ERROR] mail: could not find CKEditor translations for "de-DE"

I assume our logic to load the ckeditor translations should only take the first two characters / everything before the dash into account.