PushOwl does not currently have a built-in feature that automatically translates web push notification messages into different languages.
Your web push notification will be sent to all your subscribers in the language you craft it in. There is no auto-translate available for your Custom Prompt also. Similar to web push, your Custom Prompts will be shown in the language it is written in.
Only the browser prompt will be shown in the preferred language that your subscriber has selected. This is because the browser prompt is controlled by the subscriber's browser and will show its message according to the subscriber's language.