Add accept language header to all requests

[abelgardep]

Related Issues

App: #3980

Library PR (if needed): owncloud/android-library#551

• Added changelog files for the fixed issues in folder changelog/unreleased. More info here

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QA

[jesmrec]

With the attached fix, Accept-Language is always included by using the most generic variant of the language. That means:

Accept-Language: en
Accept-Language: es
Accept-Language: de

There is no es-ES or en-US or de-DE . Requirements are fullfiled, so it's valid.

OK from my side.

[michaelstingl]

Accept-Language: en
Accept-Language: es
Accept-Language: de

There is no es-ES or en-US or de-DE . Requirements are fullfiled, so it's valid.

Same in web implementation ✅