Are project naming conventions more important than language naming conventions?

I'm working on a project with a senior developer and he doesn't really abide by the naming conventions of the language that we're using. The project is in Go and he uses underscores for everything. For instance, in Go public fields on structs should be named something like TeamID, but in his code it's Team_id. There is already a lot of code written that follows this naming convention.

So basically my questions is, should I ensure that my code follows the naming conventions of the language or should I focus on keeping things consistent and use the naming conventions set forth by the original developer.

I spoke a little about it to him, but he doesn't really care about stuff like that and thinks it's unimportant.