I want to use some of the modules. Like forum but it's translation is uncomplete.
Now I apply to translate service to complete it and others. But if I install it now, is there a possibility to update it's language after?
(of course without making uninstall and install again to protect its contents)
Usually the translatable strings of a module can be found in the "lang" directory of a module. There should be a php file for every language that's currently translated. If your language is missing, you can just translate the contents of en_US.php.
You can do that even after the installation of the module, it shouldn't break anything.
I'm not sure I understand your question - so you have untranslated text in the forum module, right? Does that mean its not translated into your language, or not translatable at all?
You can sign up at http://translate.silverstripe.com to complete your specific language, and download the generated lang files "on the fly" - you don't need to wait for a release to do this. This is preferred to modifying language files directly, as other people won't benefit from your new translations.