Thanks, omegatron, for your quick answer!
For sure it should not be your duty to supply the language files or even to use the language phrases from different forums! PhotoPost is a product and it's a feature but not a duty that it can be integrated with different forums.
The only thing which I - an probably/hopefully many other users - would need is, that the users should be able to choose their languages out from the available languages in the gallery. So, if there is a directory /languages/english and a dir /languages/german then a visitor of the gallery should be able to set in its profile if he want's to use English or German - not more.
Of course, it would be fine if this selection would be checked in forums to avoid double selection - but as you said this leads probably too far.
And regarding the language files - every admin of an international site should be able to translate by himself the English language into his mother language or use the files which are shared here by the users.
Just a simple manuel selection where user can choose out of the available languages and store this in its profile. If one is not registered he has to choose at every visit...
Once again thank you