How to spell check a text written in a “Text Box” inserted to the document from “Form Control toolbar”? Any text written in a Text Box is ignored also after pressing F7.
LibreOffice is based on OpenOffice, thus, answer for your question you can find here and here (there is no direct and easy solution for this problem).
Oh 
it is bad. I have found a lot of similar inquiries over the web… Why they do not implement such a function? Is it impossible or something?
I think the reason is that the form control functionality isn’t popular and related tasks have a small priority. Or may be, there is no any bug/suggestion about this problem on the LO issue tracker.
@Andrej1, If you’d like to see this feature implemented in a future version of LO, Please file an enhancement bug and provide as much information as possible. The QA team will be happy to help you triage your feature request in the bugtracker. Thanks!