No alien or person who is not a citizen of the United States, or who has not declared his intention to become a citizen of the United States in the manner provided by law shall acquire title to or own any land in any of the Territories of the United States except as hereinafter provided. The prohibition of this section shall not apply to cases in which the right to hold or dispose of lands in the United States is secured by existing treaties to citizens or subjects of foreign countries, which rights, so far as they may exist by force of any such treaty, shall continue to exist so long as such treaties are in force, and no longer.