Why can a user seamlessly switch from one language input to another on the same keyboard? The question regards the phenomenon where a user can switch context on the same tool seamlessly, and as it seems without thinking. The only practical use of the phenomenon that I know of is the switching of input languages on the same keyboard, but there are probably other real world examples. 

Often these users don't type the way I do (looking at the keyboard), still they are very skilled in both Swedish, English and Icelandic keyboard setups. Is this a kind of automated process, such as the use of a piano from a professional concert pianist who does not need to think, only play with what is learned from hours and hours of exercise? I believe it is related. but not the same. 

I'm asking of what this mechanism of switching context is and why it works seamlessly for users. 

An image of the Icelandic keyboard setup: 



Image from http://farm8.staticflickr.com
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).