MapReduce is a programming model to perform distributed and parallel processing. MapReduce can be written in Java, Python, etc. The choice of a programming language depends on programmer i.e. how comfortable you are with a particular language. Though Hadoop is written in Java but you can write MapReduce in any language you feel comfortable.
I prefer writing the code in Java.
You can refer to this blog for writing the Map Reduce Example in Java: https://www.edureka.co/blog/mapreduce-tutorial/