MarkoK Creating the localization docs is next on our todo list, but I've gotten bogged down in real life at the moment (as has @Qiaeru), so they'll be a while longer. However, I can give you the short version here:
First, you should download the latest version of the English language pack ... or if you'd rather work with a git repo, you can fork it and make a local clone.
Then you need to translate all the language strings in the files contained in the files in the locale folder. There's one file for the flarum core, and one for each of Flarum's bundled extensions. If you need help figuring out how the files are formatted, you can look at the i18n docs. They're meant for extension developers rather than localizers, but they contain a lot of the information that will eventually end up in the l10n docs.
You'll also need to edit the composer.json file with the correct details about your Finnish language pack, and add a config.js file containing translations from Moment.js. Look at the French language pack for an example of how this is done. If you run into any snags, @Qiaeru may be able to help out.
Once you're done, you can make your language pack available to others on Packagist. Please be aware, however, that since Flarum is still in beta, there are likely to be many changes to the language pack that you'll need to keep up to date with.
VIRUXE why not just Fork the author's Repo and make a pull request with your changes?
Normally I'd suggest the same thing, but the existing Finnish repo seems quite out of date. You might want to ask @mhisham if he's got any plans to update the language pack in the near future. It could be that he's been waiting for us to publish the l10n docs. đ
