When an application says it supports certain languages, it often means that the application has been localised in these languages. This difference is significant, because many applications are able to support all the languages that have Unicode support.
One of the problems that people have that want to use one of these "other" languages is the ability to just state that they are writing in their own language. The Unicode enabled applications should have a list of all the languages that are supported in Unicode and thereby provide the most basic level of support. This way people are enabled to enrich their document with the right meta data for their language.
Many people would expect that all the official languages of the world are supported in Unicode. This is sadly not the fact. Brianna informed me that several official Indian languages are not fully supported in Unicode.
In order for applications to know what languages they can support on this most basic level, there is a need for a public database that keeps this information up to date. Yes, it would be great if it also includes a link to a font that is needed as well.