I opened Outlook Web access with microb , it worked but I saw tha language is German. I checked the about:config and saw that general.useragent.locale is set to de-DE which I have never set.
Then some more weird things appeared:
Changing the prefs.js by setting this setting to en-US worked but starting the browser restored it to the old one. So the data must be saved elsewhere.
This issues another question: from which file does the browser restore the prefs.js ?

Then I changed the setting in microb itself by setting it to en-US, restarted the browser verified by about:config that is was still correct and it was.
But opening outlook web access showed everything in German again but the about:config was not changed.
Very weird.
How can I set the settings so that German is no longer displayed ?
The site does not have a language setting, so it must be something in the browser which controls the language.
The same I have with another WebVPN portal site in which the text is also German. When I open Youtube the text is also German.

I cleared the cache, removed the cookies rebooted the tablet, but the problem persists.