As a Dutch (The Netherlands) user I run - for good reasons - English (US) Windows (7 and 10). I decided to switch to Thunderbird as my email client. But on en-windows my regional setting - of course - are the Dutch ones: weekdays are Maandag, Dinsdag etc. in stead of Monday, Tuesday and the same counts for month names mei, juni, juli in stead of May, June, July. See attached screenshot of Region and Language Settings. As Thunderbird Calendar (Lightning) pretents to derive the data formats from the Regional Settings i expect the Dutch names for weekdays and months in my calendar. But guess what: Lightning picks up the right long and short formats but in the OS (Win7) language, which is English. See 2nd attached file: Calendar Mozilla Thunderbird. I browsed through the about:config settings in the Lightning preferences but didn't find a setting that look appropriate to change this behavior. Most possibly I overlooked something, or it is a Lightning bug, but not matter what: any help is appreciated. Thanks for reading this and thinking about possible solutions... Kind regards, Hans Troost