I’ve changed lang from Japanese to English on macos(sierra). And I installed Thunderbird(us english version) over Japanese version. But only Thunderbird’s “Events and Tasks” is displayed in Japanese “予定とToDo”.

So I checked config editor by reading this article.

My settings are …

intl.locale.matchOS false<br /> general.useragent.locale ja-JP-mac

Then I tried some patterns like …

intl.locale.matchOS false<br /> general.useragent.locale en-US

intl.locale.matchOS true<br /> general.useragent.locale en-US

but still in Japanese.

Finally, I found add-ons. There was one Add-ons “Lightning”. And I removed “Lightning”.
Sorry, I don’t use Events and Tasks on Thunderbird. 🙂