It's mostly up to personal choice. There are official guides for JavaScript and TypeScript, Python and Java so I'd obviously suggest those for people wanting to get started with GitHub Actions.
Though, with that said, GitHub Actions are just bits of code run in virtual machines so you're free to use pretty much any language of your choosing. You can even use Docker Images (which might contain/be set up for a language of your choice) too.
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Which language should I use to create a github action? I’ve seen lots in JavaScript and there are some in Python
It's mostly up to personal choice. There are official guides for JavaScript and TypeScript, Python and Java so I'd obviously suggest those for people wanting to get started with GitHub Actions.
Though, with that said, GitHub Actions are just bits of code run in virtual machines so you're free to use pretty much any language of your choosing. You can even use Docker Images (which might contain/be set up for a language of your choice) too.