To change Pidgin‘s language from the detected one, add the variable “LANGUAGE” to the system wide environment variables.
For example for british english, add a variable “LANGUAGE” and set it to “en_GB:en”. Be aware that this will change the option for all programs which use this variable.
To set it just for pidgin you can create a batch file “pidgin.bat” like this:
Place the file in the pidgin folder, create a shortcut and use it to launch pidgin.