Fix getFixedT function always returning values in current language

[Jyrno42]

I was running into an issue where getFixedT always returned values in the current language not the requested language. I found this issue - #1184 about it. The fix for it was merged in and released and seemed to work. However a bit later the fix got reverted as people did not expect all locales to be loaded by the library. See the following issue - #1267.

This PR proposes a potential new argument to the serverSideTranslations function (could also be a config override field really). This argument can be used to provide extra locales to load for the specific page and allows one to opt into loading more or all locales. As a result getFixedT will then work for the locales that are loaded for the page.

If the general direction/idea is accepted I will clean this PR up and will add tests/documentation updates too.

Checklist

• only relevant code is changed (make a diff before you submit the PR)
• run tests npm run test
• tests are included
• commit message and code follows the Developer's Certification of Origin

[adrai]

sounds interesting... can you also adapt the readme and add a test?

[Jyrno42]

Sure I'll do that.

Do you think the extra argument is fine or should I move it into configOverride?

[adrai]

configOverride is the passed in next-i18next config... so I think the extra argument should be ok.

[adrai]

Thank you for your contribution.
Included in v11.3.0