Fix support for PO Export / Import

Problem/Motivation

It looks like PO export may not be 100% working.
According to https://www.drupal.org/project/multilangng/issues/3337232#comment-14892466 📌 Improve translate-from-non-english-source by adopting $srcLang from LocaleExtend Fixed
@geek-merlin:

Research: To stay compatible with the world-dominating PO model, we can not add arbitrarty fields.
Because the mapping (msgid, msgctx) => msgstr MUST be unique.

Steps to reproduce

Just guessing but adding the same string source for two different languages, the exported string msgstr will be the same.

Proposed resolution

I guess one of these, either:
* Add some filter on locale export to select "Source language". This if we want to export one po file per source language. Same on import page.
* Add the source language into the context on export, automatically, something like "[srclang=es]" . That must be parsed out on import. This will be useful if we want to export mixed source languages in on PO file. (Is this any use?)

Remaining tasks