Internationalization is turned off in v8 when built for node.js. The reason is that the library that provides it significantly increases the size of the node binary for a small perceived gain. You can turn it back on if you're willing to build node from source. First you'll have to check out the github repo (https://github.com/joyent/node) and then do the following from the repo root:
svn checkout --force --revision 214189 \
http://src.chromium.org/svn/trunk/deps/third_party/icu46 \
deps/v8/third_party/icu46
./configure --with-icu-path=deps/v8/third_party/icu46/icu.gyp
make
make install
These instructions are from the README.md of that repo, which can be read at https://github.com/joyent/node