I think it depends on whether you're asking about the Mac or Windows version. The Windows version requires the .NET framework, so it is almost certainly one of the .NET languages. One of the Atlassian staff members could be more specific.
I also think the Mac version is built with a more native language, but I don't run a Mac, or even know what options there are for Mac native languages.
Supported Platforms macOS Sourcetree has a lot to offer and, like many developer tools, finding and using it all can be a challenge, especially for a new user. Everyone might not love ...
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