Improve default translation using preferred keys

[msujew]

What it does

Related to a comment in #13028.

Adds support for entering preferred translation keys for the nls.metadata.json translation process. This issue in particular is seen because there are ~5 different entries that translate to File in the metadata file. The last one is translated differently compared all others. This isn't an issue usually, since all entries that use the same english value translate to the same value in any other language.

For this case, we now have a set of "preferred keys" that have priority over other keys. This currently only seems to be necessary for the File entry, but contributors can any keys in there to improve localization.

How to test

1. Change the locale of the application to something other than English
2. The "File" menu entry should be translated to whatever "File" in your selected language is. It shouldn't show the translation for "local file".

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[martin-fleck-at]

Change looks good to me and definitely solves the problem with the 'File' menu entry having the wrong translation. So we can definitely merge this, thank you Mark!

Just to make it very clear: The idea here is to have Theia maintain this list of preferred keys, is that right? Cause there is no way as an application to contribute to this list or override/extend the LocalizationKeyProvider.

[msujew]

The idea here is to have Theia maintain this list of preferred keys, is that right? Cause there is no way as an application to contribute to this list or override/extend the LocalizationKeyProvider.

Yes, adopters shouldn't really use nls.localizeByDefault (which is the only consumer of this API) anyway. Adopters should provide their own translations. We just do it because we would need to provide 1000+ translations, which are contributed by language packs anyway.

[martin-fleck-at]

@msujew Great, thank you for the clarification!