Translate Field Flag

Every widget is based on a web part. You can create widgets with the same functionality as most web parts in the system. If you require a widget with completely new custom functionality, you need to develop an appropriate web part first.

https://docs.xperience.io/k10/developing-websites/preparing-widgets-for-users/creating-widgets

Kentico default provide Translate field option for Web part property not for Widget property. If you want to this as widget property then first create property on web part and allow on widget.

Thanks Vasu, the web part has the "Translate field" checked but refuses to translate the custom widget. And yes, I did go over the Language documentation few times.

"Translate web part properties" is checked under "Translation Properties". Also, "Automatically import translated submissions" is also checked.

Google Translation API works just fine because the page gets translated with the default Kentico widgets except for the custom widgets.

I tried hundreds of times, deleted the page versions, clear the cache, restarted the server... with no luck.

You have to provide values for those fields for them to be translated in the property definitions on the widget you hover over the property's text box and it should bring up a gray plus sign in the upper right. Use that to create a new localization key and string this should get you what you need.