I’ve got some tough news for you - Career growth is (almost) never a completely straight line. You will be promoted into positions, using languages, in domain

I’ve got some tough news for you - Career growth is (almost) never a completely straight line. You will be promoted into positions, using languages, in domains, all of which isn’t “your” language or “your” domain. If you’re being asked to do it that’s because someone recognized your innate skill and ability and is hoping you can apply it to this new team. If I were you, I would not let them down. Lean in and do what you do. If it means re-tooling, then do it.