While Texas was a Mexican state, I'm going to assume the official language was Spanish, but I know that many settlers (if Stephen F. Austin's 300 families and the large immigration from the US were any indicator) likely spoke Spanish as a second language at best. All documents I've seen from the independent Republic of Texas were printed in English. Was English the official language or merely the de facto language? Moreover, roughly how many used English as the primary language in their homes during the years Texas spent as a Mexican state, independent republic, and finally as a US state?
Stephen Austin founded his colony in 1821. At that time, there were fewer than 8,000 Spanish-speaking tejanos and perhaps 15,000 Native Americans in Texas. These numbers should be taken with a grain of salt. Many of the Native Americans in Texas were nomadic and lived in Texas for only part of the year (or simply passed through from time to time while still considering it home). There were also a number of small isolated Spanish and American families and settlements who were in Texas specifically to avoid being counted or taxed (or arrested).
Within five years, Austin's colony numbered closed to 2,000 (including about 500 slaves), and was therefore already a very significant part of the population.
American colonists began flooding in during the 1820s - by 1831, Austin's colony had a recorded population of 20,000, so by this point the Americans (again, including their slaves) had overtaken Native Americans as the largest population in Texas. When Juan Almonte inspected the American colony on behalf of the Mexican government in 1834, he reported its population as nearly 25,000 - and this is the point where we can say a majority of Texans spoke English.
EDIT: Source, the Texas State Historical Association. More info on Texas' population history there.