The button used to toggle a localization preview in the UMG designer tab does not appear to have an effect on any of the text in the designer tab.
Result: The Button does not appear to toggle and does not appear to translate the text (that has been setup to translate to Spanish in this example)
Expected: The Button would toggle the deep orange shown when other similar buttons are used and it would provide a preview of the language chosen to be previewed
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-81276 in the post.