its not necessarily that the languages are declining, its that google is becoming more mainstream. the amount of language searches could be increasing, just increasing at a slower rate than non-language searches.
"Google Trends analyzes a portion of Google web searches to compute how many searches have been done for the terms you enter, relative to the total number of searches done on Google over time. We then show you a graph with the results -- our search-volume graph -- plotted on a linear scale."
"Google Trends analyzes a portion of Google web searches to compute how many searches have been done for the terms you enter, relative to the total number of searches done on Google over time. We then show you a graph with the results -- our search-volume graph -- plotted on a linear scale."