Slitherine

Maybe I am wrong, but the "New equipment types are available for purchase and/or upgrade" message box only seems to notify about new units belonging to the player nation. Is there any way to make it appear when new units become available for the other allied nations as well?

Works fine to me while playing the AK campaign with Germans and Italians:

You have to click on the Nation so they are available in the scenario.
Then they only show up at the beginning of the scenario.

I don't know where the file is for the screen prompts during the game.

It would be nice to get it during the game for all nations.

Also, it does not work in MP games.

Works fine to me while playing the AK campaign with Germans and Italians:

Ah, now I see. Italy, besides Germany, is set as player nation in the AK campaign. Meaning that Italian units purchased in that campaign will get to the core army (given that there are enough slots). Also, new units available for the Italians will appear in the message box because of it.