Can you please clarify what you mean? Do you mean user interface languages, as you can set in Admin > Settings > i18n languages?
If yes, then unfortunately it's not currently possible to add additional languages at this time. AtoM gets its multilingual support from the underlying Symfony 1.x PHP framework that the application was originally built with. As such, the list of supported languages is part of the Symfony framework and not something we can easily resolve. We have a full list of supported languages and their corresponding codes here:
If you mean adding translations to existing taxonomy terms, you can do this by:
- First, navigating to Admin > Settings > i18n languages, and making sure that the language you want to use for translation is added
- If you add a new language via settings, then you will need to reindex the site first
- Navigate to the term you want to translate (Manage > Taxonomies; find the taxonomy; browse the terms, and go to the view page for the selected term)
- Flip the user interface to your desired translation language using the Language menu
- Enter edit mode - the original term name will be shown in a yellow box above the edit field. You can add your translation and then save. Repeat as necessary for any other translation languages.
Hope this helps - if you mean something else entirely, please let me know!