@acw I'm sure someone has already mentioned the "all major browsers must support $LANGUAGE, or it won't fly" argument. Way back in the IE6 days, Microsoft added a second scripting engine to their browser ( #VBScript ). Over time, I stopped seeing or hearing about it in a web context, though .vbs is still used for Windows admin tasks.

Browsers are slowly dropping support for plugins, but a scripting engine for whatever language where the engine was loadable by non-admins as a browser extension could possibly build some momentum if influential sites built desirable content that used it.