CF Page Rule newb - need guidance

Background
We host our translated web structure on a proxy server and use subdirectories to host these translations:
e.g. site.com/fr site.com/nl etc

I have created two rules to forward all traffic to the proxy server but only one of them is working:

site.com/fi/ this has a resolve override and a host header override implemented and is working lovely

However we need to also cover site.com/fi (without the trailing slash). I cannot use site dot com/fi as this will then limit us from using additional root level structures that start with /fi in the future (e.g. /financial-software or /final-countdown)

I did try site.com/fi$ with no luck and site.com/fi\$ - but still no luck.

Is there a way that I can create the rule and still keep the option open to use full words in future page URLs on the example above?