Texas has several requirements for the issuance of a marriage license, but residency is not one of those requirements.   Your fiancé does not have to be a citizen in order for you to get married in Texas.  There may be some logistical issues, though.  If your fiancé lives in Mexico, she will need to make plans to come to the county where you intend to be married at least 72 hours in advance and go apply for the marriage license with you.  Generally, both applicants must appear and present proof of their age and identity (like a birth certificate and driver’s license).  There are some exceptions to the requirement that an applicant appear and present this proof, but the situation you describe does not fall within one of those exceptions.  Every county clerk has a form available in their office.  Before you get to far along in your planning, drop by their office and obtain the application for a marriage license so that you will know exactly the requirements and filing fees for the county where you intend to be married.