A helper class forming a hierarchy of supported language codes, via nested classes.
All language codes are eventually represented as string constants. This is simply
a code-convenient form of the table at https://cloud.google.com/speech/docs/languages.
It is regenerated regularly, but not guaranteed to be complete at any moment in time;
if the language you wish to use is present in the table but not covered here, please use
the listed language code as a hard-coded string until this class catches up.