Although RFC3066 language tags work well much of the time, there are still some issues:

• Many more codes are needed than those provided by ISO to cover the approximately 6,000 languages of the world.

• They don't cover the needs to express general regions; for example, there is still no tag for the generalized Latin-American Spanish that many organizations use to create Spanish content.

• There is some lack of clarity between the use of language tag values for designating language vs. locale. 'Locales' are combinations of language plus geographical region typically used to set such things as date and time defaults in software.

• There is a need, sometimes, to distinguish the script used, in addition to the language. For example, Mongolian might be written in Mongolian script or Cyrillic; Croatian might be written in Latin or Cyrillic; ...

People are currently working on solutions to these issues, including people from ISO TC37, SIL, and W3C, etc. The proposed successor to RFC 3066 is also targeting these issues.