This is an alternative to #15340 (it works with the Chain interface; see: #15340 (comment)).
Refs:
- #16218 (partial fix)
- #15894 (comment)
This PR:
- makes GUI error messages bilingual: user's native language + untranslated (i.e. English)
- insures that only untranslated messages are written to the debug log file and to
stderr(that is not the case on master).
If a translated string is unavailable only an English string appears to a user.
Here are some examples (updated):


qt5ct: using qt5ct pluginmessage is my local environment specific; please ignore it.
Note for reviewers: InitWarning() is out of this PR scope.

