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I tried to install the dictionary again. This time I did not get an error message, so I clicked on 'install'. However, the language option does not show up in the email. Can you please tell me what is wrong?

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Sorry to hear that. On my end it seems to properly install in 60.4.0 and 65 beta.
Are you sure you have spellcheck enabled in Options > Compose > Spellcheck?
You also might try to uninstall the add-on properly, restart Thunderbird, go to Hamburger menu > Add-ons > Dictionairies > top-right search on addons,thunderbird.net and type "woordenboek" and install again.