A language's ISO code should be according to the ISO 639-1 codes list.
The Subscription Suite offers a few built-in Base Languages. If one of those matches the language you wish to add, you can choose it from the Base Language dropdown to have most of the Subscription Suite texts pre-translated. Afterwards you just need to translate missing texts or change the wording where it is different from the base language. Translations are done on the Translations page.
Select the default language by checking the Default checkbox in the row of the respective language.
Activate a language by checking the Active checkbox in the row of the respective language.
Remove a language by clicking the Remove action. The respective language is deleted without further warnings.
Add a language
Add a language by entering the language's name into the Enter a name input field. Click on the Add button.
The language name will be visible in the portal's language selection menus. Remember to supply the language's name in the native language and don't translate it. For example use "Deutsch" for German and "日本語" for Japanese. Otherwise, if you translate language names (like "English" to "英語") users who accidentally set their UI to Japanese will not find their way back to English.
After translating to the newly added language please don't forget to activate it - see next section.
Activating a language
While a language is not fully translated it should not be activated. After adding a language (see above) it should be first translated under Languages. Once a language is translated it can be activated by setting the Active checkbox in the language's row.
Configuring the default language
If a different language should be the new default language you can configure that by checking the Default checkbox in the language's row. Please don't forget to remove the Default checkmark from any other language.