Translations are defined in JSON files which are located in the main git repository here. Each of the files correspond to a language and is a JSON object where the keys are the English language strings and the values are the translations to the relevant language.
Many of the languages are machine translated and have not been validated by hand. To fix one of the translations, fork the git repository, edit the JSON file and submit a pull request back to the main repository
To contribute a new language translation, fork the repository and add your JSON file to the folder. The name of the file should be one of the two-letter language codes listed here.
Then, go here and add a line to the code for your new translation, like this:
[2 character code]: "[this language's name for itself]",