Why can a user seamlessly switch from one language input to another on the same keyboard?
Focusing on the word "seamlessly", in the more general case (not just typing)--once you look at cognitive/linguistic activity with a fine enough resolution it becomes clear that language-switching is not usually seamless.  There is typically a cost (lowered performance, either in speed or accuracy) to switching languages in any modality, which is typically greater from second language to first language than from first language to second language (Language Switching &amp; Competition, Second Language Research April 2012 vol. 28 no. 2 131-149, among many others).