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.
If you have stuff thats not translatable in the first place, have a look at http://doc.silverstripe.com/doku.php?id=translation and http://doc.silverstripe.com/doku.php?id=i18n#collecting_text