[KevinH]

Just use Sigil to open the original epub. You can then run SpellCheck dialog in multiple languages at once (if the xhtml properly used lang attributes). And yes Sigil uses the same hunspell spellchecker system that LibeOffice uses. And it comes with both English and French dictionaries.

And you can run epubcheck as a plugin and use it Validation Window to find and fix errors.

Alternatively, you can use any unzip tool to open any epub. Then you can edit the files to your heart is content. Then use Sigil's FolderIn plugin to load the epub's files, run the epubcheck plugin, find and fix errors then save the epub properly (mimedata file uncompressed but stored first the remaining files added).

But just using Sigil to do the spellchecking and editing is easier.