I didn't find any similar RFC, but just a discussion topic of Älphäbet url support
Since October, 2009 ICANN has approved the creation of internationalized country code top-level domains (IDN ccTLDs), and first of then were installed in the DNS root zone on May, 2010. Now IDNs are actually in use already.
Thus, support for IDN (which is a part of IRI - International resource identifier) seems to be a mandatory for any web application, including bulletin board.
It would be nice to implement it as soon as possible to make it included into 3.1a.
Some useful links:Internationalized domain name
- WikipediaInternationalized Resource Identifiers (IRIs)
- Internationalized Domain Names in Applications (IDNA): Protocolrfc5892
- The Unicode Code Points and Internationalized Domain Names for Applications (IDNA)
EDIT: The derivation of valid characters in terms of Unicode properties for IDNA2008
- Internationalized Domain Names (IDN) FAQ
EDIT2:Unicode Technical Standard #46. Unicode IDNA Compatibility ProcessingUnicode Character Database (txt file)