Hi @AB-User ,
AFAIK, in an offline environment, you will have the ability to enable the check spelling feature for a target language if you've already installed the target language in Windows. Edge has a built-in support for huge number of languages even in an offline environment, so you don't have to install language packs for Edge. I suppose you can install language packs for Windows to enable the check spelling feature. If it does not work, please let me know.
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Best Regards,
Shijie Li