|traduire_nom_langue is applied to the
#LANG tag and returns a translation of the language code provided by that tag (fr, en, it, etc.) into that language.
Remarks: The translation of codes are made into the language that that code represents, and will follow the writing conventions of that language.
As such, "fr" will be translated as "français" with a lowercase "f", where as "es" will be translated as "Español" with an uppercase "E".
If you wish that this filter returns content with uniform cases for the first letter, as you may wish for section headings, for example, then you can pass the results to the PHP function