Complex sentence: The Mexican government does not categorize its citizens by ethnicity, but only by language-so statistical information about the Nahua only deals with speakers of the Nahuatl language although unknown numbers of people of Nahua ethnicity have abandoned the language and now speak only Spanish.
Split into simpler sentences: The Mexican government does not categorize its citizens by ethnicity, but only by language. Statistical information recorded about the Nahua deals only with speakers of the Nahuatl language, although unknown numbers of people of Nahua ethnicity have abandoned the language and now speak only Spanish.