We've all heard some sort of foreign music where the performer uses another language that isn't English. That can be Rammstein, Nena, Sigur Ros, Clannad—anything like that. And yes, we can all go and look up German or Gaelic or whatever language it is, but the fact of the matter is that unless you're a fluent speaker, you can't decipher the lyrics in real time. The vocals go from lyrics that tell a story, to an instrument that makes a sound—mouth music.
Mouth music isn't a new term. It existed years and years ago in Scotland and Ireland when all musical instruments were banned, and continued to see use after the ban was lifted. In recent times, and for our use, it can be likened to scat singing in jazz and other genres of music. Lyrically, mouth music requires no set lyrics or words, but could be improvised sounds over a prescribed melody. These vocables are nonsense, gobble-di-gook that sounds good but has no real meaning.
In recent times the most prominent users of mouth music would include Cocteau Twins and their singer, Elizabeth Fraser—who is notable in her tasteful and melodic use of singing complete and utter rubbish that elicits an emotional response. Crazier than that, Icelandic band Sigur Ros created their own language—Hopelandic—where they used a code-breaker system to write their lyrics, where each nonsense word was given a consistent meaning throughout their body of work.
In playing the bagpipes I have seen what could be called a form of mouth music—it's a practice called canntaireachd. Canntaireachd is a series of prescribed Gaelic words, each one individual and unique, to certain notes as well as ornaments and embellishments. It can be sung to students, without the need of an instrument, in order to teach them tunes—in this case, usually piobaireachd.
Songwriter Roger Radcliffe (from the Disney classic—One Hundred And One Dalmatians) would have us all believe that you should write the melody first, before you write the lyrics of a vocal part. In a way he's right. If you write a memorable, catchy, heartfelt melody it means that no matter what language it's sung in, or what audience in what country listens to it—it'll connect musically with anyone.