(No comment on the German in the existing entry).
It may be that the EU or some international organizations use the term authentic language, probably influenced by French, thinking they are using English words correctly when they are not. It would be like adoption of the word Handy, seemingly English, for the English name of a certain German device, or using the word actual
Authentic has a meaning in English and legal English, and it is not what such international organization types seem to think.
See its use at http://thelawdictionary.org/authentic-act/
Definition of AUTHENTIC ACT (Black's Law Dictionary)An act which has been executed before a notary or other public officer authorized to execute such functions, or which is testified by a public seal, or has been rendered public by the authority of a competent magistrate, or which is certified as being a copy of a public register. The authentic act, as relates to contracts, is that which has been executed before a notary public or other officer authorized to execute such functions, in presence of two witnesses, free, male, and aged at least fourteen years, or of three witnesses, if the party be blind.
Also at http://thelawdictionary.org/interpretation/
Note that the word "language", when used in regard to contracts, laws and treaties, it doesn't normally mean language in the sense of English or German, but rather to refer to some (not necessarily all) of the specific text (words, sentences, even punctuation).
My advice: don't use the term "authentic language" except in a narrow context of matters relating to any treaties or international organizations that use it as a defined term (and, if you must, as in #2).
For that purpose, I suggest something more like official, agreed or authoritative version.
(For authoritative, official or binding regulation of the use of English or most languages, lots of luck!)