The other side of the discussion about whether American English is threatened by immigration is whether English threatens other languages. This has been an ongoing topic of concern for people in many countries. The French, for example, are well known for their struggles to keep the French language pure, and you have probably heard of Franglais. Here's Wikipedia's treatment of the term and its usage, followed by an online article published in March on how English seems to be infecting not only French but also Spanish.
The issue of English as an imported tongue, displacing native languages, is a concern to speakers in Africa too. I ran across this article in the Tide News of Nigeria which points out that by making English the national language, the government devalues Igbo, which is spoken by millions of Nigerians.
Interestingly, 2008 has been designated the 'International Year of Languages' by the United Nations. I am sure I will be paying attention to the developments of this novel celebration, and we should have much more food for thought about all languages, including English.