From HEXONET Wiki
An internationalized domain name (IDN) is an Internet domain name that (potentially) contains non-ASCII characters. However, the standard for domain names does not allow such characters, so internationalized domain names have to be converted into standard ASCII domain names while. This is done by converting them into so-called punycode.
Please note, that only some TLDs support IDNs and some only support specific character sets. Have a look at the IDN category for a list of IDN capable TLDs and the corresponding TLD info for the supported character sets.
When registering IDN domains you have to specifiy the so-called language tag, which states what language/character set the domain name is in. There a different kinds of language tags:
API SupportThe API parameter
The following commands accept the parameter: