Brion's point about template link table corruption is a good one. That would indeed cause problems.
Yes, but I wonder: How different would that be from the current situation? I can say something like {{It is {{#time:s}}}} which tries to include a template – which one is considered on render, but the templatelinks table is updated only on save. So that after the first action=purge, you have the same kind of corruption. (See [[testwiki:Parser functions vs linktables]].)
(Agreed, it is not that big of problem currently, because the use cases for this are rare, while the {{USERLANG}} would IMHO spread very quickly, esp. on Commons.)
-- [[cs:User:Mormegil | Petr Kadlec]]