concrete5 in your Language
If you use the official download, you'll get a version of concrete5 that currently contains 15 languages. You can select one of them during the installation.
In addition to these languages, you might find a customized version managed by the community, check the community groups to see if there's version for your country: worldwide community groups.
Additional language packs
Some languages aren't shipped by default because they aren't completely translated or because translation changes were made after a concrete5 release. You can find the latest translations on this site translate.concrete5.org.
Contribute to concrete5 translations
If you want to help translate concrete5 to another language or help maintaining an existing translation, head over to translate.concrete5.org.
If you're a developer looking for more information about how the translation system works, check the following documentation: documentation.concrete5.org/developers/concepts/localization.