We accept punycode usernames and render appropriately; or pass through UTF-8 usernames. The biggest issue is autocomplete; so we provided that using aliases/petnames for connections. Probably will have issues on some platforms.
Might want to use a custom taxonomy/tag type for cross-posts. That's what we use for saving messages in "folders". While we don't typically federate these, I would imagine you might want to share those tags with software that supports cross-posting.
We accept punycode usernames and render appropriately; or pass through UTF-8 usernames. The biggest issue is autocomplete; so we provided that using aliases/petnames for connections. Probably will have issues on some platforms.