“I am a native speaker of Xlang. I have spoken Xlang all my life, and it’s the language I think in. If I take a proficiency test of speaking, shouldn’t I get the highest possible score?”
It ain’t necessarily so.
DLS follows the US government in measuring proficiency on the ILR (Interagency Language Roundtable) scale. The ILR has ratings from 0 to 5. Many people assume that a native speaker would automatically score 5, at least in speaking.
In fact, the ILR rating 5 is to be understood as the speaking proficiency a well educated, highly articulate native speaker. This does not mean that advanced degrees are required, but for those cultures where higher education is widespread, an ILR 5 means being able to use the formal, careful style that educated people use in some contexts, as well as the relaxed, colloquial style used in everyday life.
Someone who speaks a dialect that is considered non-standard, and cannot switch into a standard dialect when the situation requires, is likewise not rated ILR 5. In the USA, for example, there are many dialects that a public speaker or TV announcer might use, but there are others that would be considered inappropriate for a broad audience. This is not a matter of one variety being “better” than another in any way; it is simply a fact about the society where the language is used. This social attitude towards some dialects is probably unfair and certainly unscientific, but it cannot be ignored in testing speaking proficiency.
A speaker at ILR 5 will also have a broad and deep understanding of the native culture at all levels. References to a famous Shakespeare character or a well-known popular song will be easy for an ILR 5 English speaker to use at the right moment. Of course nobody knows all corners of a culture, and there will be differences between generations and countries (e.g. USA and UK). Even so, an ILR 5 speaker will have many of these cultural ducks in a row.
An ILR 5 is the kind of person who might be chosen as a spokesman for a group because of his or her ability to make a point clearly and persuasively. This is the meaning of the “highly articulate” part of the definition. There are quite a few native speakers who are well educated, intelligent, and highly competent in their fields, but apt to become tongue-tied when asked to speak about a complicated subject without much preparation. Such people are not rated 5 on the ILR scale, even though they may be more qualified in other ways than people who are rated 5. A language test, after all, is not a test of a personal worth or character, any more than it is test of specific knowledge. It is a test of the ability to accomplish things using the language, pure and complicated.