Language specifiers (used in the $text query operator, the to-be-deprecated "text" command, the text index "default_language" option, and in document language annotations) will be able to use the following aliases (in addition to their respective English-language equivalents): da, de, en, es, fi, fr, hu, it, nl, no, pt, ro, ru, sv, tr. These aliases will be treated as case-insensitive.

Original description:

Instead of strings like "english" or "spanish" or "french" text search languages notation should be standard two letters (ISO 639-1) like everywhere else (web pages, domain names, keyboards, manuals...).
That's the way it is coded in any decent application. It's THE standard for languages names. The full list here: https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes

At the very least MongoDB should accept BOTH notations. It's such a quick fix.