Add welcome.properties for about:welcome localization

[luixxiul]

Describe the issue you encountered: Add welcome.properties for about:welcome localization.

• Extra QA steps:
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• Any related issues: make welcome screen show up on first time run #9423

[bsclifton]

The welcome page itself is part of https://github.com/brave/brave-website which should allow for localization 😄

[eljuno]

Since the page is already live, maybe make an announcement on Transifex to increase localizer awareness?
I'm sure not everyone know that the strings for about:welcome is on brave-website project not brave-laptop project. And the early translation is done by vendor.

Personally, I still thinking it'll better to have it as welcome.properties on brave-laptop than separate project. Since it's a feature in browser itself. 😄

Or maybe no need to localized this? Since first time users will have Brave in English (no option to change language when first install).

[luixxiul]

Since first time users will have Brave in English (no option to change language when first install).

FYI that issue is tracked on #6435