Fixes #1479: always translate to the last requested language

[El-Ahmed]

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The current behavior:

The language change event would fire whenever a new translation was loaded. When requesting a translation, if another translation is loading, the system would wait for it to finish and return that instead of the translation of the most recent set language.

This Pull Request addresses this by:

Fixing the issue mentioned in #1479. Adding a test to ensure the fix works as expected.

[adessilly]

Is it going to be merged soon ? This fix would be very welcome ... Thx to @El-Ahmed

[CodeAndWeb]

Fixed in @ngx-translate/core@16