Spell engine
From MoodleDocs
Location: Settings>Site administration>Plugins>Text editors>TinyMCE HTML editor
The TinyMCE HTML editor used by Moodle has Google Spell as its default spell engine (spell-checker)
It is possible to change this on the TinyMCE HTML editor settings screen to PSpell or PSpellShell if you prefer. However, these need to be supported in your installation.