An oracle that gives you a predicted "source language" for every node
in a DOM, which is used when converting the result back to the source
language during round-tripping.
This code is unique to Parsoid; the PHP implementation does not
round-trip. Do not confuse this with PHP's (soon to be deprecated) method
LanguageConverter::guessVariant(), which is a heuristic used to
disable language conversion when the text is guessed to be already
in the desired variant.