If I was laid off due to the fact that I’m not bilingual, is this illegal?

It may be legal:

1) First, language is not protected in all states; that is, in many states, it is legal to fire someone due to the languages he or she speaks (or does not speak). For example, while federal law protects someone from employment discrimination due to his or  her race, it does not protect on the basis of language or national origin. Some states will protect against discrimination based on language and/or national origin (which may apply in your case), but not all do this--you need to check the law for your state. (You can repost your question with your state.)

2) Second, even where language is protected--so no firing simply due to a person's language--there is an exception when a certain linguistic ability is required for a job. For example, if you worked in marketing, sales, or outreach (e.g. social programs) to a Spanish-speaking community, then you could be terminated for not speaking Spanish; in that case, being able to speak Spanish is as much a requirement of the job as would be having a driver's license for someone working in deliveries. So if bilinguilism is a legitimate job requirement, then  you could be laid off for not being bilingual, even if you are in state which normally protects against employment discrimiantion on the basis of language.