I installed Version: 6.1.0.0.alpha0+ (x64) Build ID: 9c8a6a4396ebbe8ecb15591cc291c388c82c8ed6 …
I works great, except for the spell checker.
I can’t get it to work.
The spell check box pops up, and says “Spell check complete”, but not even intentionally misspelled words get noticed.
Spell check was working on previous versions.
I need advice.
Thanks.
Alpha0+ may be a developer prerelease and thus be installed in a different path and with a different user profile (placed ‘LibreOfficeDev’ instead ‘LibreOffice’) as compared with a released version. You probably don’t have the needed files there. If you are sure that’s not the cause of your issue: Report a bug to this site, please.
(Bughunting session for 6.1.0.0.alpha1 6.1.0.0.beta1 is announced for 2018-05-28. That’s a monday, strangely. Reports on alpha0 will probably not be prioritary. )
The bughunting session will be for 6.1.0.beta1, we are getting closer to release 
Thanks! (I actually knew, and this was the reason to post the appendix. Next second I used the wrong reference into my internal cache. Buggy software between my ears, and nobody cares for debugging. Sigh.)