Display of other language instead of english, fails

I was able to download other languages and to select them in the various places where the language could be adjusted.
Unfortunately, the display language stays on English. No onscreen error occurred.
I already tried to clear the cache of the browser, but that made no difference.

I tried several platforms, webservers and browsers. No combination succeeded in changing the display-language.

Does anybody have a hint of what might be wrong?

Your help is highly appreciated.