True, “mage” is not “Magi” in German, but they are not translating the word “mage” to German. There are two kinds of skulls you can get from the Chapter 2 encounters.
One of them is called sorcerer’s skull in the English version. You get it by killing sorcerer NPCs in camps and it looks like this:
The other is called Magi skull in the English version. You get it by killing the sorcerers from special encounters (for example, Baalzamon in the volcano), and it looks like this:
That’s the one you’re showing in your screenshot.
The word “magi” is the plural of the Latin word “magus”. These words originally referred to Zoroastrian priests and those of earlier religions from the same region. They are also the origin of the words like “magic” and “magician”.
In modern usage, many fantasy authors use the word “Magus” and its plural “Magi” to refer to wizards, sorcerers, or magicians of higher status, to distinguish between a regular wizard and someone who has dedicated their life to pursuit of studies of higher forms of magic.
Conan Exiles follows the same pattern: the NPC you encounter all over the map are called “sorcerers”, but the handful of special bosses are called “Magi”.
(As an aside, I think the skull should be called “Magus skull” in the English version, but I can understand wanting to avoid confusion.)
Now, I don’t speak German, so I would leave it to you, @Testpionier, to answer the following question: what would be a good German translation for “sorcerer” and “Magi” that would preserve the distinction between the two?
For that matter, I’m curious as to how they translated “sorcerer” to German. Did they use “Zauberer”? If they did, then it would make sense to use “Magier” as a translation for “Magi”.
