When there are characters other than Hebrew letters as well as characters other than Hebrew letters in the text, the entry in the cell edit area (next to the = sign) is distorted. The letters either don’t appear at all or some do and some don’t. There are boxes where letters should be.
Can you please write what is your LibreOffice version and also what is your operating system and name.
If you get squares instead of real characters, then you most probably have a font problem. Try changing the font to some font that supports characters you would like to use.
Well, that depends on the paragraph direction mostly, can you please add screenshots?