Signs aren't free. That's the big deal.
Nothing is stoping foreign-language groups from printing up and distributing materials in their own language on using the Metrorail system, or any other mass-transit system.
Yeah, well the reality is we have a lot of Spanish-speaking people and we have signs in English and Spanish. Nothing is about to change.
They also take up alot of space.
Imagine having even 10 languages on 1 sign/set of signs; try finding as you drive by exactly which 1 will tell YOU in YOUR language in a timely manner how to proceed!