Admittedly RFC 2070 is old, and its status is historic. But it was the first HTML specification with I18N based on UNICODE, and the last HTML specification published by the IETF.
So far its DTD had to be extracted manually from the RFC, now IANA hosts an official master copy with the public identifier urn:ietf:params:xml:pi:-:IETF:DTD+HTML+i18N:EN. The urn:ietf:params:xml:pi registry was created by RFC 3688 for DTDs developed by the IETF. Of course HTML i18n is still SGML, not XML, but its DTD is now the first registered IETF DTD.
If you have old HTML i18n documents you can use an URL of this DTD as system identifier like this: