How to switch the frontend language?

Hi,

a) I installed the french pack in addition to the standard english.
b) I set default to french
now everything in the frontend is french. fine.
c) I set default to back to english
the frontend stays french!
empty cache: still french
restart browser: still french
uninstall french: frontend still french !!!

so far everything Firefox

d) start Opera
frontend english > good.

e) reinstall french and set as default
frontend stays english (in Opera)
f) restart Opera
frontend french

in Opera the language switches after a restart.

In Firefox the frontend language stays french, whatever I set in the backend, even after emptying the cache and a restart!

Any ideas?

Thank you!

Cheers, Ralf.

ok. I just found out that I can switch languages by clicking on the flags in the front end. That doesn't explain the strange behaviour described above, but for the time being that's ok.
Cheers, Ralf.