so in silverstripe 3.0, silverstripe is pulling translatable out of the core and turning it into a module which i am excited about because it means we dont have to hack the core to do certain things.
I created a module that silverstripe is looking to merge into their translatable module associate domains to specific locales. it was originally built to handle different top level domains (like .de, .fr, .jp, etc), but i modified it so it should be able to handle subdomains within one tld as well. there isnt any support for sub directories like you are looking for, but for now its something. we havent incorporated it into the translatable module yet because it needs some beta testing, and i didnt add it to silverstripe's module list because it will probably be merged into translatable in the future.
also, i included directions on how to set up multiple virtual hosts to simulate multiple domains while you are developing.
you can download the module at https://github.com/cbryer/Translatable-Domains
if you get a chance to kick the tires, please let me know how it goes, or if you have any questions.