Oh, the language aspect gets a lot worse than that. They explicitly have a non-goal of "all grammatical features of all languages", but the "common" cases are hard enough. From https://github.com/unicode-org/message-format-wg/blob/main/s... :

  .local $hasCase = {$userName :ns:hasCase}
  .match $hasCase
  vocative {{Hello, {$userName :ns:person case=vocative}!}}
  accusative {{Please welcome {$userName :ns:person case=accusative}!}}
  * {{Hello!}}

But if anyone can find a good compromise, it's the Unicode team.