In Unity, the main programming language is C#. C# is widely used in Unity, and it is also the programming language recommended and mainly supported by Unity. Almost all of Unity's official documentation and tutorials use C#. Unity no longer supports programming in JavaScript/UnityScript and Boo languages.
C# is a type-safe, object-oriented programming language that combines the power of C++ with the ease of use of Java. The syntax of C# is clear and consistent, and it supports advanced features such as generics, delegates, events, Lambda expressions, LINQ, etc., making it easier to write complex game logic and systems in C# in Unity.
In addition to C#, Unity can also support some other programming languages through plug-ins or extensions, such as Python, Lua, etc., but these are usually not mainstream choices, and using them requires a higher technical threshold, such as understanding how to communicate between Unity and other languages interaction, and how to deal with performance and compatibility issues that may arise.
If you are a Unity developer, I recommend you to use C#, which will allow you to get the most out of Unity's features and find tons of tutorials and references.