Show a quickpick for changing display language

[Rachel Macfarlane (RMacfarlane)]

While verifying issues I had to change my display language a couple of times, and I think the experience for it could be bit better.

Instead of opening the locale.json file, this PR uses the localization service to get the list of available languages and display it in a quick pick. The current language is selected by default, and the placeholder text explains that VS Code will restart on selection. When something is selected, the locale.json file is written to as before and VS Code gets restarted.

When running out of sources I don't ever see the UI language change - I haven't dug into how nls gets loaded so I'm not sure if that's expected or not. But I do see that the locale.json file is updated correctly.

This fixes #22758

Let me know if you think this is a good change. Thanks!

[Dirk Bäumer (dbaeumer)]

Rachel Macfarlane (@RMacfarlane) thanks a lot for the PR.

[Dirk Bäumer (dbaeumer)]

I looked over it and I have a suggestions: I think we should indicate whether the corresponding language pack is available locally. If not it might even be cool to install it. Otherwise the expierence might not be nice when selecting a LP that is not installed.

[Rachel Macfarlane (RMacfarlane)]

Ah, thanks! I had misread the code for getLanguageIds. I've changed it now so that the quick pick is only populated with languages that are available locally, and there's an option at the bottom to install more languages that opens the extension's viewlet with a query for language packs.

[Dirk Bäumer (dbaeumer)]

Rachel Macfarlane (@RMacfarlane) have you updated the PR. I think I don't see the new code changes.

[Rachel Macfarlane (RMacfarlane)]

Dirk Bäumer (@dbaeumer) it should be updated now, I just rebased against master

[Dirk Bäumer (dbaeumer)]

Rachel Macfarlane (@RMacfarlane) LGTM. Feel free to merge in.

[Rachel Macfarlane (RMacfarlane)]

Dirk Bäumer (@dbaeumer) Thanks for reviewing! I pushed one more change to show a confirmation dialog before restarting, based on feedback when demoing to the Redmond team. I'll go ahead and merge since that addition is very simple.