Try the following steps:
1. Right click on Wargame: European Escalation in the Steam library -> Properties -> Language. Select one.
2. Ingame, look for 'Opcje' and click on it --> then go to 'język' --> change it from 'polski' to 'english' --> click on 'zastosuj'
3. Enter the Wargame game directory inside [...]\Steam\steamapps\common\wargame european escalation\ and open the file install.dat. Change the number 685234 to 199285. Don't forget to save the file.
4. Uninstall the game, chose english in the steam game language options and then re-install the game.
If it still doesn't work, contact support at https://www.eugensystems.com/contact-our-support/