Push notification translations is supported for ServiceNow provided OOB languages and also for unsupported (self localized) languages. Steps for apps team to add translations OOB and also for custom notifications created by customers.

• If its a custom language, Add the custom language to the instance - https://www.servicenow.com/docs/bundle/yokohama-platform-administration/page/administer/localization/task/self-localize.html
• Set the language preference on the instance Preferences —> Language &Region
• Then add the all the keys to sys_ui_message_list.do
• Push Notification Script - Use the gs.getmessage method in the push notification script to fetch the message key. The gs.getmessage will fetch the message from the table based on the user's language preference. Sharing the article link .
• Example Script
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