JanusGraph can be queried from all languages for which a TinkerPop driver exists. Drivers allow sending of Gremlin traversals to a Gremlin Server like the JanusGraph Server. A list of TinkerPop drivers is available on TinkerPop’s homepage.
In addition to drivers, there exist query languages for TinkerPop that make it easier to use Gremlin in different programming languages like Java, Python, or C#. Some of these languages even construct Gremlin traversals from completely different query languages like Cypher or SPARQL. Since JanusGraph implements TinkerPop, all of these languages can be used together with JanusGraph.