To change your app's language, open More, then go to Settings. There, you'll find Language. Tap Language to see a list of available options. Tap on your preferred language and confirm the opened dialog. You can change it at any time.
It's important to note the distinction between your Eventee app's language and the event's language. You control the language of your mobile app, while the event organizer sets the language for the event itself. For instance, you might set your app to Spanish, but the event content will still be in English if that's what the organizer chose.