hey jarmo,
there are actually 3 ways to generate your own language files while I would highly recommend to only use the last one (overrides).

1) transifex
you can download all the files that you want to change, then edit them to your liking. after that's done you need to rename them (because by downloading from transifex they're changing the filenames a little bit) and at the end copy them to the right folder.

2)easysocial sync
in the easysocial backend you can find a language tab. there you can automatically grab the latest files from transifex, these are copied over to your site, renamed and added to the correct folder. from there you could go on with your editing while the steps of renaming files and putting them to the correct folder were already taken care of, so you saved a big amount of time

3)language overrides
by using joomla's native overrides function you can add all your frontend overrides to one file and your backend overrides to another file and keep them in one place. this has many benefits as it's by far the fastest way as you don't have to edit 100 single files but keep everything grouped together. also in case of updating your changes don't get overridden, so you don't have to re-do them every time a new version is out.
here you can find some further informations about using overrides: http://stackideas.com/blog/how-to-override-joomla-language-strings

hope that helps but if you have any questions feel free to get back to us at any time. have a nice day