Let me tell you three situations that have me thinking about this.
First, if you are trying to catalogue or create metadata for an item that is not in your native language, can you complete the task? Would it be helpful to know another language? We know that some languages have similarities, so could knowing one additional language actually help you navigate a few more than that? And would it make you a more effective librarians?
Second, if you are working the reference desk in a city that has a diverse population, should you be able to service people in their own language? In some industries that diversity in language is sought and valued (e.g., hospitals). Should libraries also seek to have that type of diversity on their staff?
I have a student who is doing an internship in a public library. He has realized that being conversant in Spanish would be a good thing. In the U.S., a growing segment of our population speaks Spanish, so shouldn't our library staff speak Spanish? (And if there is another language widely used in the community, shouldn't we have staff members that also speak that language?)
Third, if you are build a service (e.g., digital collection) that will be used by a diverse group of people, would it be helpful to have text in their own languages that would help them use the site? Would you want to outsource that work? Would you want someone on staff to do that work or even know the language well enough to be able to supervise the work?
If you agree that knowing a foreign language would be useful for library and information science graduates, how do we encourage them to learn a language or maintain fluency in a language? Should we ask existing staff to learn a language that is being used in the community and even tell them which language they need to learn? (For example, you need to learn Mandarin, not French.)