I am sorry but at the present time none of the Cyrillic languages are supported.
It is a problem, I need to have it in slovak language - is there another posibility to have submit button with different text?
Not for the HTML form. The label of the button is not customizable.
If you generate the PDF form (paid subscription) and have Adobe Acrobat (not Adobe Reader) then you can do some modifications in Acrobat to the PDF before distributing it (including chaning the label of the submit button).
I tried to edit the "Submit" text in my generated PDF form and got an error message. Within Adobe Acrobat XI, Tools panel > Content Editing tools, when I click the Edit Text & Images button, an error message appears: "This is a secured document. Editing is not permitted."