Tahoe 26.2 language reverts to US instead of GB

In addition to Tom's suggestion, System Settings -> General -> Language & Region is worth checking to see that everything is as you want it , particularly Preferred Languages at the top, e.g.:

With an erase and reinstall last week, I also noticed that detection of a UK location sets US English by default.

Use + or – as necessary.

A340driver wrote:

the language is still reverting to US instead of GB.

Please provide more details. Are you talking about the keyboard mapping? The spell check/autocorrect behavior? Something else?

What do you have on the list at system settings / keyboard / text input / edit / all input sources ?