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.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.