The --config argument wants a path to the rainlendar2.ini file instead the ini file so you would need to create a separate folder for the settings file. I.e. create a folder called "ES" under the .rainlendar2 folder and copy the rainlendar2.ini file to there. Then start Rainlendar like this: C:\Program Files\Rainlendar2\Rainlendar2.exe" --config "C:\Users\Tombuctu\.rainlendar2\ES\"
Alternatively you can also make multiple portable installations
of Rainlendar. That way each of them uses different settings so you can set a different language for each of them.