General Questions About Royalty and Monarchies

[Somebody]

I've always to know, why the royals are called in French "Gotha"? Ex.: "mariées du Gotha", etc.

It's not specific to France but originated in Germany. The 'Gotha' is a reference to the Almanach de Gotha (or in German: Gothaischer Hofkalender). This book about royalty and the high nobility was first published in Gotha, Germany, hence the name.

See for more information the wikipedia page on the Almanach de Gotha.