The term less commonly taught seems to have been borrowed in the HLT community from the second language teaching community where it refers to instruction within a specific target market, be it the United States, the Western Hemisphere or perhaps outside of the region where the language is official.
notwithstanding the number of native speakers, if most of those also speak another language with an even greater population or prominence, that could reduce the language's importance to projects whose goal is to develop news understanding technologies.