Another thing to consider while adding Java to a Grails project is usually fine, because most people who know Groovy, also know Java, adding Kotlin might not be a welcome addition. If you are the only one on a team that knows Kotlin, then you would be the only one who could maintain it, which might not go over well, and if you ever left that project that would be seen as legacy code to be replaced. So I would think do you have a good reason for adding Kotlin, or even Java, over the default Groovy. Now on the other hand if it's a personal project, or a project where you are delivering an end product, i.e. not the code, and you're the only one maintaining the code, then there is no real downside to adding whatever language you like.