Respect region settings!
When there are settings in the OS for how to display and format dates, time and other stuff, then use it!
If you want the user to be able to override the settings in the OS, that's OK, but then it should be an Opt-In system for that, and it should be at least as good and flexible as the settings in the OS.
Don't reinvent the wheel, use the design guide that Microsoft publish on how to write software for windows.
Pratik Dave commented
Default language should be utilised from OS configuration as per comments below or allow msi switch to install local language.
Jóhann Áki Björnsson commented
Probably the best way to support other language formats that are not like the on on offer for teams for now (like no Icelandic language on offer and date and time formats not supported)
Use OS settings for desktop app - calendar, date/time and numbers format
A specific example: the message timestamps shown in chat history are displayed with US dates, even though my computer has Australian regional settings.
Victor Sorokovikov commented
I do want to have settings from my OS!
George Gooding commented
The default language setting should come from the OS.
For overriding these values, you should be able to set the language and the time/date formats separately, like you can in the OS.
For example I usually have all my computer programs in English, while I use the time/date formatting for my country (Norway).