Über dieses Add-on
The dictionary needs to be defined in the "Custom 4" field of the contact in the address book.
"Custom 4" can be found in the "Other" tab of the contact.
You need to set the language code as used by the dictionary.
Examples: en-US, en-AU, en-GB, de-DE, es-ES, it-IT, fr-classic.
Please ensure that you get the language code correct, otherwise it will not work
(and there won't be an error message).
The add-on notifies in the status area of the message compose window when it sets the dictionary.
This is a "no frills" add-on. No configuration, no options.
Note that one day in the future Thunderbird will absorb this add-on, since there is an enhancement request for it:
With this add-on you get the solution today!
For Thunderbird 60.0 please visit the "All Versions" section to download a compatible version.