Personally, whatever programming language would be chosen,
I really like Aryeh's approach to sanitize and "compile" the
template to PHP. It could be used everywhere MediaWiki runs,
it is no hassle to set up compared to installing other in-
terpreters and the performance should be the top of what PHP
has to offer (and *all* templates could be compiled to that
code). From a distance, I think it would even be easier to
have the file/memory/CPU restrictions hacked into the main
PHP interpreter rather than to cook our own soup.
Come to think of it, it would also fit very well with pro-
filing individual templates.