Explain underlined.:
Language in the world - the clearest, the most logical; that we must guard it among us and never forget it, because when a people are enslave, as long as they hold fast to their language it is as if they had the key to their prison. Then he opened a grammar and read us our lesson. I was amazed to se how well I understood it. All he said seemed so easy, so easy! I think, too that.
It means, if people of a country are being treated as slaves by the people of the other country which invaded them and are in power, if they hold on to their language, they still have the key to their freedom and independence. He is just trying to emphasise on the fact that holding on to one's language is essential even if one is enslaved.