Why is the spell checker not working in 3.5.4?

I installed 3.5.4 and discovered that the spell checker is no longer working. I was forced to uninstall it and re-install 3.5.3 to get the spell checker working again.

Reported as → Bug 50552 - SPELL check does not mark wrong text and capitalization rules
…and a lot of duplicates…

See also my answer → LibreOffice 3.5 spellcheck not working.

Try that → workaround.