Hello:
Open the speech settings dialog (with the shortcut for your language or opening the NVDA menu with NVDA+n, then Preferences submenu, Settings, and pressing down arrow to find the corresponding panel).
Then tab and ensure that the checkbox to detect language switching is enabled. In this way, the combo box to configure the announcement about language not supported will be available.
Press tab to focus the corresponding combo box, and select the desired option, for example, disable the announcement.
Hope this helps.
And merry Christmas if you celebrate it.
Noelia