1. Download a recent Rockbox source code package.
2. Get the file named rockbox/apps/lang/english.lang
3. Save it as [new language].lang (where new language is the
name of your language!)
4. Add your translation on all lines that start with "new:". Don't change or edit any other lines.
------------
To actually try/use your new translation, you MUST have perl installed. Then you run:
perl binlang <english file> <language file> <output file>
binlang is a script found in the tools directory of the Rockbox sources.
Then, when newer Rockbox versions come with more strings to translate, there's another handy tool that'll make life easier for a translator:
perl uplang <english file> <translated file>
uplang outputs an "updated" version of your file. It adds all new strings that should be translated, and it adds notes if one of the previous english phrases have been changed since you did the translation.
Phew, if this is not good enough. Please go ahead and try and ask if you get problems.