Karl Eichwalder wrote:
Then go for art-tp.wikipedia.org, but that's cryptic and tp.wikipedia.org is simply misleading and sooner or later 'tp' will become an official ISO-639 language code. These two-letter codes are precious.
To quote Brion: 'tp' is not assigned as a 2-letter code, and ISO isn't assigning any more 2-letter codes. Rather than fight about what to call it, I figured 'tp' will do. -.-.-.-
Which I remember stated in a document too. Maybe what's suggested in the RFC (1766 and 3066 if I recall the numbers correctly), we should use the x primary subtag, and name it x-tp to avoid trouble (such as them getting a 3-letter code). Same goes for Lojban and Ido and Klingon which don't seem to have codes yet either.