Hi Tomas,I use Transifex, Transifex, Localise, Crowdin and Weblate. Weblate allows custom installation and has a glossary, a solid API. We used Weblate years ago experimentally to translate Koha into English, but since Pootle was used, it was difficult for us to maintain the translation in another tool. Weblate also has the ability to translate plurals, which is a fairly essential feature for some languages.
Localise has the most options, including auxiliary translation tools (DeepL, Google translate, etc.), but it is not free and not open source.  Transifex is similar, but has a fairly straightforward interface.
I vote for Weblate and will be happy to abandon Pootle, as it is no longer an efficient and pleasant tool for making a translator's job easier. And Koha has a lot of text to translate.

Michal