I just read Mary Beth Raven Blog entry about the new Notes 8.5.1 spell checking engine.

So if you use the Standart 8.5.1 Client the new engine will use the:

com.ibm.langware.v5.dic.en_.... folders with AU..., GB..., US..., US_IBMTerms... and fr_CA which can be found in your Notes\framework\shared\eclipse\plugins directory instead the old *.dic files.

If you prefer the old spell check engine you need to add SPELL_FORCE_LEGACY=1 to your notes.ini.