why the voynich ist celtic

[Petrasti]

why the voynich ist celtic

No, it isn't  
Voynich Manuscript is a medieval work showing Italian and German influences. Connecting it to Celtic folklore which comes mostly from British isles is a very long shot.

In the same way I could connect it to Slavic folklore, take some random text about rusalkas and fern flowers and add it to Voynich pictures.

Of course - Celtic, Germanic, Slavic, Roman, Greek and so on share the same core as they are ultimately part of the big European culture. But I don't see in VM anything that would be exclusively Celtic and lets say not Germanic.

I don't rule out a German influence in general; there is much to be said for locating it in northern Italy or Switzerland, or possibly southern Germany. I think it's quite possible that the writers were also accustomed to writing in German. If you look at the words in the Voynich manuscript, many of them begin with a “k.” If it were an exclusively English or Irish work, we would probably find a “c” instead (but I am not an expert on this, it is just my personal opinion, but it would be interesting to explore). If I understand correctly, the attached link to slide f85r2 is the basis for the assumption of a German origin. Of course, these could be the seasons, why not, and of course the images could also be similar to images in German manuscripts. Whether the writers were German, Italian, or even Scottish, Irish or Valaisian cannot be explained by this. We speak more than one language today, too.
The seasons also feature in pagan beliefs. They are particularly honored there. However, I read an article, which I unfortunately can no longer find, in which the woman in the south with the crook and the “rosary” was attributed to English manuscripts.
The rosary, which is atypical due to its three rings, was described in this image evaluation as “cloud bands,” which in turn were attributed to England. Is it not surprising that there is no reference to a known religion in the Voynich manuscript? 
And yes, of course you can also attribute the images to Slavic folklore, if you then include and explain  handfasting, the dark and light full moons, the offerings to the plant spirits that were practiced in Indo-European culture with honey and mead, the absence of any known religion,the naked women, the plant devas who hold the stars in their hands and find them reflected in the star section (or flowers) If you take a look at my thread with my proposed solution, you will find some more details that point to a Celtic origin. However, if you find any Slavic folklore or similar that solves the mystery, please let me know :-)