Hi,
Some comments:
1: The "multi-lingual support" was more a question, than a feature request, as I understand there is little to no interest for multi-lingual captioning (except maybe in my home-country Switzerland, or Belgium). I don't know of any non-proprietary software that supports multiple concurrent languages.
2: I had the idea for multi-lingual support because I have developed a web photo database that translates captioning information using machine learning into the language of the system user, for example translating and displaying an English caption automatically into German, when the user has set his/her preferred language to German. If the user downloads a photo, I replace the caption information (typically written in English) with the translated one. This works fine, but has two drawbacks: a) I end up with multiple versions of each image, one per language, and b) the end-user only sees the translation, but not the original language.
3: The XMP specification includes multi-lingual support (Ch.2 and Ch.3, Language Alternatives) , but is not very clear (? does it only apply to rdf:Alt elements, which would be insufficient, as other blocks, like keywords, also contain local data).
4: I have create an example of an image of multi-lingual caption stored in the XMP data using my understanding to the XMP specification.
5: I can read the modified file using PhotoMechanic and Photoshop, the two applications I use.
5.1: PhotoMechanic discards non x-default language information in rdf:Alt blocks when displaying the data, and ignores the xml:lang tag in other blocks, e.g., in rdf:Bag. When saving caption data, all non x-default data in rdf:Alt blocks and all other xml:lang references are discarded. This is unfortunate :-(
5.5: Photoshop discards non x-default language information in rdf:Alt blocks when displaying the data, and ignores the xml:lang tag in other blocks, e.g., in rdf:Bag. Photoshop fails to correctly save the XMP data altogether.
6: From an implementation point of view, I would be less pessimistic. I believe to implement multi-lingual support, it could be sufficient to have on the Metadata (IPTC) Info and Metadata (IPTC) Template pages a drop-down box that allows the user to select the language in which to display/edit the data.
7: Conclusion: As long as major image processing software don't support (at least passively, i.e., not discarding multi-lingual information) multiple concurrent languages, I don't see any use to follow-up on my idea, especially as only having one software supporting it (and the others "destroying" it, by removing the multi-lingual data when saving) would not add any value! :-(
Thanks anyway.
Claude