Kotlin is a very tame language choice, in what I assume is a very Java heavy organization. Peers should be accountable for learning new languages and following the implementers rule.
it's critical to root out these hero types lest they ruin your organization with hand grenades wrapped up as "refactored" projects using the latest language. seniors won't put up with this, and rightfully so: it basically raises a middle finger to the organization's previous culture/work, and they are now responsible for doing it... again. it's disrepectful and ought to be a fireable offense, ideally rooted out in the first few weeks when the new team member is given some slightly autonomous tasks.
Good thing for you, that excellent software is always written at the bark of a sadly decomposing mind. You're making wherever great again, I suppose.
Mandate the standardization of interfaces rather than languages and you empower all types of developers in your organization.
Unfortunately letting something like this sit means you have a huge time bomb of tech debt that only one person can work on. It is a liability for the company and long term profitability.