Language Recomendation

I’d like some help.

I’ve written a prototype in JavaScript / HTML.
It’s now time to convert it to a working, marketable, product.
I’d like some recommentation on which language, products to use
along with pros and cons.

I want the user interface to be on the Windows platform.
1) I’d like some recommentations on which language to use and why.

It needs to “talk to” OS/390 and zOS where I have some code running.
The OS/390 code is BAL (assembler).
2) I’d like some recommentations on which language/product to use to send data back and forth (Windows – mainframe) and why.

Thanks in advance.
John