Hi Sven,
If I am getting your problem right, the only choice would be to buy licenses directly from Parallels. It is a kind of technical limitation - upsells on core Plesk functionality cannot be bought separately from base license keys, since they require re-issueing base key with new properties. Known examples are "PowerPack" and language packs. Obviously we would have no right to modify the license key of your hosting provider, so the only solution would be to cancel your current license and buy new one - now from Parallels. I must warn however that the price may be much different. Who is your provider BTW?
Since it is totally up to providers what they sell, we are going to address the problem on our side to unblock such upsells. However re-working licensing core will take some time and thus cannot be expected really soon.
Regards

Hm, here is what I see in our online store on 2nd "next"
Language Packs
- 1 Extra Language
Plesk comes with one-included Language. Add more to fully localize the Plesk Panel UI and help into any of: Brazilian Portuguese, Chinese Simplified, Chinese Traditional, Dutch, English, French, German, Italian, Japanese, Polish, Russian, or Spanish.
$4.99 for 1 Month
- 2 Extra Languages $9.98 for 1 Month
- 3 Extra Languages $14.97 for 1 Month
- 4 Extra Languages $19.96 for 1 Month
- 5 Extra Languages $24.95 for 1 Month
View attachment 7316
It is before checkout and questioning for contact information. Does it answer your question?