Go through this list and follow the instructions given marked in dark red. When you're done, press the "Finish translating" button at the bottom of this page. You will then be sent a patch file with your changes. Submit this file to Rockbox on the patch tracker.
If a field is read-only, it means that this string is not meant to be translated and you should leave it as-is. The special string "none" is not meant to be translated, so if you see it in the english version, put "none" in the translation as well.
When you click this button, you will be sent a Rockbox language file. If you are satisfied with your changes, you're encouraged to submit this file in the Rockbox patch tracker.