By all means - you should learn a different language. To quote John Searle: "You can never understand one language until you understand at least two.". I personally feel that knowing several languages has expanded my understanding of all of them, made it easier for me to communicate in any of them, and made me a better person (including a much better programmer).

I personally am not sure I can recommend any language. Hebrew is my mother language (being an Israeli Jew), but it's kinda useless except for Biblical/Mishnathic/etc. research, because most Hebrew-speaking Israelis have working English. I like Hebrew a lot, and find it a wonderful language, but it is kinda hard and as you know, not many people know it (yet).

I've also studied Literary Arabic (or Written Arabic) for 6 years. It's a beautiful language, but very difficult, and counter-intuitive, even for a Hebrew speaker, and Arabic suffers from a very severe diglossia, and most Arabs are not literate. I've spoken with two Arab Israelis who've studied both Literary Arabic and Hebrew, and they both said learning how to read and write Hebrew was easier for them than learning Literary Arabic. Since then I've lost most of my vocabulary.

I also studied French for 3 years in Junior High School. It seemed likeable and nice, but I was told it gets much worse as you study more of it, because there are much more exceptions than words that follow the rules. French is naturally very useful.

Spanish is also very useful, and arguably the easiest language to learn, and I don't know it very well. I was told that it makes learning other languages much harder after one learns it.

See also what I wrote about why Chinese may not become the next international language