Yesterday i changed the language of the forum from Deutsch (Sie) -> Deutsch (Du) and was no longer able
to browse the forum. Only a blank page appeared.
Deleting history allowed to access board index.
@rockedge did reset the language and login works again 
Thank you so much.
@HerrBert I downloaded a fresh package of the German (Du or Casual Honorifics) language pack and re-installed it.
We'll need to test it though and it might be a problem with the language pack itself. Hopefully the re-install will have fixed it.
It's not there 
Acceptable - if it doesn't work, don't offer it.
All well with Sie - but if we ever meet RL, i would like to say Du 
KUDOS
@HerrBert I had to remove it! I logged in as a test user I use for these occasions and selected the (Du) version and I had the blank page. I was able to use a different browser (Opera) and I could load the page to get to the admin to switch it back.
Now I am thinking of enabling it again and trying with a different browser. I also think something with the session cookie is involved with causing it not to work. If you are willing to test it again I could always reset again!!
Let me know so I can enable it for the test. 
but if we ever meet RL, i would like to say Du
selbstverständlich
One thing before we go on testing:
The issue occurred using Min browser. After changing my settings, i was no longer able to browse the forum
with firefox and palemoon after login either.
...
[edit] Around 21:10 UTC Tried again with Min - no change. Switched to firefox (115esr). Cookies and history deleted.
Deutsch (Du) still not available in settings. Deutsch (Sie) works as expected.