What language did they write laws in? What language did Congress debate in? What language did the average person speak in day-to-day life? Today they speak English but they used to be a part of Mexico, so did they suddenly decide to speak English and forget Spanish at some point?
The Republic of Texas was founded by American immigrants, whom Mexico had lured in by free land grants and tax breaks (coupled with freedom of religion and a blind eye to slavery.) By the time they declared independence, they were a significant majority of the Texan population. They continued to speak English, both in daily life and in the government. While the Mexican minority probably continued to speak Spanish among themselves, they never had a significant influence in government.