What kind of front-end? Something that runs in a browser? Something that runs on a desktop/laptop? Something that runs on a mobile device?
If you're not super-concerned about performance (and you probably shouldn't be at this point) then you can do number crunchy stuff in most any language. Especially if what you're doing is mostly proprietary. If you're doing "standard" things for which libraries exist then you'll probably want to pick a language that can take advantage of existing libraries.
If you're okay with a language that requires a run-time environment then I'd give Python a look. It's popular in the scientific community for number crunchy tasks that have a non-command-line front-end.
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www.scipy.org]