The email client Thunderbird and the Internet browser Firefox can do spelling checks if a dictionary is installed. Dictionaries for multiple languages can be installed.

Usually a dictionary is installed by opening Add-ons menu, then in the search field write the English name of the language, e.g. German for Deutsch, to search for a dictionary.

For unknown reasons there is no Swedish dictionary to be found, even though this used to be available through the normal add-on menu search.

Nevertheless, it is possible to install a Swedish dictionary.

How To: Swedish Dictionary in Mozilla Thunderbird and Firefox

SwedishDictionary_ThunderbirdAddOns2