1. A language model back-off system for controlling user interaction with a user interface employing plural language models, selected from the group consisting of N-words, N-characters and combinations thereof, comprising: a navigation constraint module employing plural probabilistic language models, including a first language model and a second language model that provides a lower level of constraint than the first language model, to constrain navigation of selectable user interface input components each corresponding to a corresponding one of characters, so as to constrain the characters that a user is allowed to input, a user input interpretation module receiving user input and initially interpreting the user input using a level of constraint favoring the first language model to determine if a selection of a next character to be input is made of one or more user interface input components, wherein said user input interpretation module, if the selection is not made, determines whether conditions are met for backing off the language models employed to constrain navigation of the user interface input components; and a language model back-off module backing off the language models to change the level of constraint by favoring the second language model over the first language model if the conditions are met, wherein the user input interpretation module and the language model back-off module are implemented by controller.