If you want to contribute with a translation, it’s very easy using transifex.
First go to www.transifex.com and register as translator (free).
Then go to
The first link contains the translations for smplayer, the application, while the last one allows to translate the smplayer website.
Click on the language you want to help to translate and select “Join team”.
Once you have been approved as a team member, click on the resource you want to translate.
A new screen allows to work on the translation online, although it’s also possible to download the translation to work on it offline, and later upload it back.