If it’s just a thought and not a phrase or words that has to be translated or anything … and an image that everyone could understand … I’d go with the standard …

Imagine everyone all over the world seeing this thought all at once and everyone confirming what they saw among themselves and everyone else everywhere … everyone confirming the same image across languages, countries, regions, religions, rich, poor, literate, illiterate.

A bonus would be to have the alien say something in a language and voice that absolutely no one on the planet could possibly understand.

Then repeat the broadcast for five seconds once a day, every day.