Computer Configuration > Administrative Templates > Microsoft Edge
Both
HKLM\Software\Policies\Microsoft\Edge
Enables different languages for spellcheck. Any language that you specify that isn't recognized is ignored. If you enable this policy, spellcheck is enabled for the languages specified, as well as any languages the user has enabled. If you don't configure or disable this policy, there's no change to the user's spellcheck preferences. If the 'SpellcheckEnabled' (Enable spellcheck) policy is disabled, this policy will have no effect. If a language is included in both the 'SpellcheckLanguage' and the 'SpellcheckLanguageBlocklist' (Force disable spellcheck languages) policy, the spellcheck language is enabled. Example value: fr es