Allow to override language metadata

[msujew]

What it does

Closes #11010

1. Fixes a bug in the LocalizationProvider which should've allowed to merge localization. It only merged the translations property and nothing else, leaving the languageName empty.
2. Returns a bit more metadata in the getAvailableLanguages() method.

How to test

1. Change the language and assert that the app works as expected.
2. Set a breakpoint in this line and assert that the languageName and localizedLanguageName property is correctly set in the availableLanguages array (is undefined on master).

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

[vince-fugnitto]

I confirmed that getAvailableLanguages now contains additional information:

However there is an error thrown when executing configure display language with a language-pack installed:

[msujew]

@vince-fugnitto I cannot reproduce the issue you experience (the error message in the console), but I've made the code a bit more error-safe (it now checks that the languageId is actually a string). Does it still happen for you?

[vince-fugnitto]

It works well for me now 👍

For reference I had:

• installed the french language pack
• triggered the configure display language command
• the quick-pick was empty, and the error was thrown