I couldn't find a setting for default dictionary. Try Setting an Alternate system language instead. Under the Thunderbird Menu choose Options or Preferences Click on Advanced on the left side Click on the General tab at the Top Under Language set the Drop down to Dutch and click the Set Alternatives button. Set Dutch at the Top and English below that.

I think it would, the picture looks promising. But I can not find (or recognize !!) the Thunderbird Menu where I have to choose Options or Preferences. Apologies !!

Under Tools/Options(or AppMenu/Options/Options)/Composition/Spelling, select the default dictionary in the Language drop-down.

Default dictionary has been changed successfully. Thanks to both of you. Regards Lange99

This still leaves the question how the unwanted change of the default spelling dictionary (from Dutch to English !!) could have taken place, without my involvement, knowledge or consent ?? Does anyone have an idea ?? Could it have been part of the last update (version 68.4.1) which was downloaded and installed automatically middle of last month ??

Had this poblem too, with recently installed a 2nd language dictionary I only use occasionally (TB 68.4.2). The English language normally used for email is "downgraded" relative to the newly installed language pack. However, with solution given as shown above, I am now back to English as the default language when composing email. What a relief!

These are good solutions, but they don't work for Canadian English. Because Thunderbird does not recognize *Canadian English*. There is a Canadian English dictionary, available as an add-on, and it's good, but it does not get loaded in the dictionaries files for T-bird, and so cannot be set as default. My workaround is to set UK English as the default, and then when I spell check, to select Canadian English. but it's tedious to have to do that. Why doesn't Thunderbird recognize Canadian English?? (dunno :-{ ).

I don't have that problem here. See the attached pictures that show EN-CA as the default dictionary. Which language is your version of TB?

So, the trick is to go into Options, then Composition, then select Canadian English.

thanks!!

For Thunderbird 68.7.0 (linux) to change the default language do the following: In thunderbird main window click on the three horizontal line icon to the right of the menu bar aka "Thunderbird Menu" then: Preferences/Preferences/Spelling Select your preferred language from the Language pull down menu.