I created a dump with mongodump on computer A (ubuntu 12.04 server). I moved it to computer B (ubuntu 12.04 server) and typed:
mongorestore -db db_nam
Exporting LC_ALL="en_US.UTF-8" only works if you have the en_US locale installed. If you want to avoid installing the locales package (or its equivalent on distributions other than Debian derivatives), then you can instead use:
LC_ALL="en_US.UTF-8"
en_US
locales
export LC_ALL=C.UTF-8
which will not require any extra locale data.