You can go to Tools > Options and click on the spellchecker tab.

1. Using MS Word dictionaries – Checking the “Use MS Word spell checking” configures SDLX to use MS Words dictionaries. This only works with whatever language your installation of Word is.

2. Using additional installed dictionaries – You can try downloading the .dic files of different dictionaries and see if this works.

We have found the following links for your reference:
Spell Check Dictionary Winedt
Spell Check Files Karamasoft