Doc: Translation Guide
CyanogenMod is constantly adding new features, but those features are usually initially available only in English. We are trying to have those features available in every supported language however.
You just need to create an account on http://translate.cyanogenmod.org and request to become a translator for your language.
If your language is not supported in AOSP, you also need to join http://translate.cyanogenmod.org/project/cyanogenmod-aosp in order to complete all the system translations.
You do not have to join the translator community to correct translations. If your language has active translators, you can file a bug report using JIRA.
There is a dedicated Google+ community for CyanogenMod translators. Participation in this group is mandatory if you would like to be involved with translating CyanogenMod to your language. This is largely to ensure you are informed of recent code changes that affect translations. Send an email to the translation lead, Michael Bestas, if you are interested in joining.