Copy en.php and save as xx.php -- where xx is the language ID you are translating into, e.g. sv.php for Swedish. Visit the IANA language subtag registry if you don't know the language ID. Always use the macro language, e.g. the sub tag for Mesopotamian Arabic is acm, but we just say ar to indicate it's Arabic.
Open the file in your favourite text editor and update the @language tag. Don't touch any other @tags and keep the file structure exactly AS IS. Translate the strings and save as UTF-8 (without BOM) with Unix line endings.
Submit a request to the repository if you want to share and include your new translation. However, you should be prepared to maintain your translation and keep in sync with future updates before posting a request. Abandoned language files will be purged.
If you're a native speaker of any of the existing language files and spot any typos or errors, please report your suggestions, thank you.