Shouldn't Problem Solving - Data Structure round be language independent?

Had my 2nd round (PS-DS + Android Fundamentals) with Swiggy for the aSDE role. The interviewer gave a string-based question. After some discussion, I came up with a better, though not optimal, solution. He then asked me to code it in Kotlin. I’ve never practiced DSA in Kotlin, so I asked if I could use Java instead. He said no, wanting to check my proficiency in Kotlin. In the first round, the Machine Coding round, didn't they check my proficiency in building apps using Kotlin? Despite doing well in my Android Fundamentals part and my first-round Machine Coding round, I received a rejection email today. Not sure what these startups expect from freshers.