Language Codes
The language codes use the CLDR (Common Locale Data Repository) format string that uses a two letter language code (e.g. "fr" for French) optionally followed by a two letter country code (e.g. "fr-CA" for French in Canada).
If an unsupported language code is used, then the service will attempt to fall-back to the closest available language. This is done by stripping regional and extension subtags to find a known language code. For example, French Canadian (fr-CA) is unsupported so this falls back to French fr.
Should the fallback fail, then the service will return category names in the default language en for English.