And Prolog is actually an ISO-standardized language so you have a choice of implementations to choose from (for example Yap Prolog, which I found much faster than SWI last I checked, or the highly regarded commercial Sicstus Prolog). Plus, implementing a Prolog engine isn't too much of an effort to do on your own (like I did two times). In fact, there are multiple Prolog implementations for the JVM, for JavaScript, etc.