[horde] German translation not working in certain browsers

[Stefan König]

Hello List,

i have installed IMP H3 (4.3.7) on Ubuntu 12.04 LTS and have a problem
switching to the German translation with IE and Chrome (have not testet
more browsers yet).
Firefox works like a charm, I can switch between en_US and de_DE on the
login page but IE and Chrome (new or old versions like IE 7) refuse to
switch and show only English text.

Am I missing something here? I have been messing around with this for
several days now and am clearly lost.
Any help is highly appreciated!

thanks
Stefan