You may want to look at this solution on this and some recent changes: Error in Rhino 8 SR31 when using pandas: unknown locale: en-US [7:1] - #18 by eirannejad
The fix in Ehsan’s fix_locale.py module, posted in the thread (Error in Rhino 8 SR31 when using pandas: unknown locale: en-US [7:1], post #18). It both corrects the BCP-47→POSIX name and overrides locale.getlocale so it won’t throw. Put it on the Python search path and import it before pandas:
python
import fix_locale
import pandas
Note: an important detail is that just adding the setlocale line to their already-crashed session won’t help — pandas is cached broken. They need to reset the Python runtime (or restart Rhino) and ensure the locale is fixed before the very first pandas import, ideally via fix_locale.py or a ScriptEditor startup command.