Well given the Chinese constitution at the time (which is the only document we have describing the state theorized in this thread - see my second link in my post) it would be quite likely that any Republic of China that survived through the 1940s into the 1990s on that basis would probably:
1. Not have a problem maintaining Portuguese in one small municipality returned by Portugal
2. Constitutionally such cultural autonomy is permissible and likely even protected.
3. Might not even be able to stop a Macau municipality from adopting Portuguese as a local language