Who became a mexican citizen, learned spanish, and convinced the mexican government to appoint him as an empresario so he could sell land to american colonists in texas

You are referring to Stephen F. Austin (1793-1836), the so-called "Founder of Anglo-American Texas," who took on his father's plan of settling families in Texas as a land agent (or empresario). He willingly became a Mexican citizen (and insisted that his colonists became citizens too) and he started by bringing it three hundred families. That number increased significantly, but in 1834, on his way back from Mexico City, he was arrested and imprisoned, accused of inciting insurrection in Texas - an accusation that was altogether false, since, unlike other fellow empresarios, he had never wanted Texas to secede from Mexico. Upon his release, however, he was determined to confront the Mexican troops that were advancing towards Texas, and he was elected commander of the Texans’ volunteer army. Texas eventually gained independence from Mexico in 1836, and although Austin was candidate for president of the new republic, Sam Houston became president, while Austin was elected secretary of state.