Copilot Chat (accessed from Outlook) is in foreign language different from M365 profile and app language. How to change?

Question

Hello,

a client of mine reported this issue where their interface buttons in the Copilot Chat (such as teh Work-Web switch, the prompt composition box, the "new chat" button, etc) appear in Romanian, despite his app and profile being set up in Italian (as can be seen from the search bar at the top which says "Cerca"). To be noted, they work in Italy and do not speak Romanian, so there is no reason why it would be set in this language.

We checked and the rest of their Outlook app and other M365 apps are in Italian, so how can these interface buttons be changed?

PS. The prompts examples in English are just fine and not a hindrance, but the interface buttons in Romanian make use of the Chat difficult as my client doesn't speak the language.