How to change Voiceover language for an activity on MacOS

Hi, I'm new to Mac and activities. I have setup an activity for the RS Games Client for the Mac, but I can't figure how to change the language of this activity, cause in fact, the main purpose of creating this activity is because I'm a Spanish language user and the RSG client is in English, so I would like that every time I open it my VoiceOver changes to the RSG activity and therefore to an English voice.
I don't see any option to change the language when editing the configuration for the activity in the VO functionality..
Thanks.