[harry8]

There are 2 choices of language for any side project.

1) the language you know best

2) the language you most want to learn

Only you can decide on which has a higher priority.

[graystevens]

This. If it’s a side project that is intended for self-development and learning, try it in a new language and see how you get on. If it’s something even remotely important or think might turn into more than just a side-project, go with what you know or are comfortable with.