The full list of languages supported in Smartcat and their respective codes can be found here. You can use the codes not only to make API requests but also to import TMX files, create translation memories using Excel and import & export XLIFF files.
Note: Different CAT tools might use different language codes. This is particularly important when you import TMX files that might have been created outside of Smartcat. If the wrong language codes are used in the TMX files, Smartcat will generate errors and not import the segments.