C++, C#
C++ is the standard for video game design. C++ is more complicated than Java or Python however is used by Unreal and Unity. C# is used on Windows and Xbox, it can also be used in Unity.
Java
Java is the most versatile coding language however it is not commonly used in video game development, it is however used with online web games.
HTML5, CSS3, and SQL
HTML5, CSS3, and SQL are all used for web programming. Therefore they are also used for online videogames.
For all of these coding languages there are resources on the internet for people to use and learn the coding languages.