Handle language tags that indicate phonetic IPA transcription The BCP-47 registry defines a variant subtag "fonipa" that can be used in combination with arbitrary other language tags. For example, "rm-CH-fonipa-sursilv" indicates the Sursilvan dialect of Romansh as used in Switzerland, transcribed used the International Phonetic Alphabet. http://www.iana.org/assignments/language-subtag-registry/language-subtag-registry