Is the discrimination?

Usually, in situations of "English Only", an employer must show that speaking English is necessary to conduct business.

In your situation, did the job advertisement specify that you had to speak Spanish for the job? If so, then the employer can probably show that it is necessary for conducting business. Perhaps the bank has a large customer base of Spanish-speaking clients.

If the ad never mentioned anything about speaking Spanish, then there could be discrimination. But you would have to consult with a lawyer in Texas experienced with this kind of discrimination claim to see if you have a case.