Colleges understand that not all high schools offer a wide variety of courses, including foreign languages. While taking multiple foreign languages is impressive, offering only one doesn't automatically hurt your application. Colleges will look at the context of your situation. If your high school only offers one foreign language and you've excelled in it, demonstrating proficiency and a commitment to learning, that will be viewed positively. They'll be more interested in what you *did* with the opportunity available to you than what wasn't available.
However, if you had the opportunity to take other enriching courses or pursue independent learning in a different language, and chose not to, that might be viewed less favorably. The key is demonstrating initiative and maximizing your learning opportunities within the constraints of your school's offerings.