Chore/add warning on invalid fallback lang

[AlemTuzlak]

I've tried to implement the warning for the issue outlined in the ticket below, although I am not sure about the following:

1. How do I test this and where? There are a lot of test files and I don't know what the best place is and what to use (mocks, spies etc)
2. Is this if check okay or could I have used optional chaining? I am not sure so I just checked if it exists and has a length
3. should the "dev" be ignored?
   react-i18next:: i18n.languages were undefined or empty react-i18next#1803

Checklist

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

Checklist (for documentation change)

• only relevant documentation part is changed (make a diff before you submit the PR)
• motivation/reason is provided
• commit message and code follows the Developer's Certification of Origin

[coveralls]

coverage: 96.09% (-0.1%) from 96.199%
when pulling 168e300 on AlemTuzlak:chore/add-warning-on-invalid-fallback-lang
into f2a6b6c on i18next:master.

[adrai]

1. Create a new test file and pass in a logger dummy like here: https://github.com/i18next/i18next/blob/master/test/runtime/i18next.hasLoadedNamespace.test.js#L4
2. if check is ok... no optional chaining
3. why should dev be ignored...

fallbackLng can be a string an array or an object: https://www.i18next.com/principles/fallback#fallback-to-different-languages

[AlemTuzlak]

Some tests pass "dev" as a fallback even though it's not in supported languages so I wasn't sure if there was some behavior I'm not aware of, okay will fix what you just said!

[adrai]

Some tests pass "dev" as a fallback even though it's not in supported languages so I wasn't sure if there was some behavior I'm not aware of, okay will fix what you just said!

ok, keep the exception with the dev language

[AlemTuzlak]

so I found something interesting, while trying to write the tests I actually got 2 logs from module called languageUtils that warned that the default language is not in the supported languages list, it seems to me that the feature is already there, maybe it's not called in the react package?

[adrai]

totally forgot that... yes... it's already there ;-)

Yes, it should be called also via react-i18next...

[adrai]

Just tested here (change the fallbackLng accordingly): https://codesandbox.io/p/sandbox/react-i18next-http-example-forked-n4fqct

seems to work:

So this PR can be closed, right?