BCP 47 language tags are built from subtags in the IANA Subtag Registry. This tool helps you find or look up subtags and check for errors in language tags.
See https://r12a.github.io/app-subtags/
This was moved from the r12a.github.io repo. You can find the previous history at https://github.com/r12a/r12a.github.io/commits/master/apps/subtags