I am not a native english speaker. If you see User profile settings defined by StackExchange, it does not contain primary and secondary languages. It just contains location but a non-native english speaker may live in a country where its language is english. Now with English language and Usage StackExchange site, it would be a nice idea To create Two more settings, maybe called primary language and secondary language. I hope StackExchange support this issue.
My impression is that the powers that be are somewhat hesitant about tweaking the StackExchange engine for beta sites, and I can't really blame them for that. Who knows whether or not a particular SE site will survive its beta at all. (Before anybody panics, we are faring rather well at the moment, just as most of our siblings.)
Now, your feature request actually isn't limited to English Language and Usage. Listing one's first/second/third/Xth language could prove useful on many SE sites, even established ones such as StackOverflow. However, I am fairly certain that a request for two additional user profile fields will get rejected nonetheless, on the grounds that every user already has an "About me" field where they can share any bits of information they please.
Note that many don't even use that field. Some don't fill out their user profiles at all. Others provide incomplete, incorrect or misleading information. And that's okay, they are free to do so, it's their personal page. The point is, adding two more fields to the database would result in having two more fields that are — at best — mostly ignored, or — at worst — completely useless. (I know that I, for one, wouldn't fill them out.)
That being said, regulars have a fairly accurate understanding of what other regulars' mother tongues are. Answers and comments speak louder than "About me" fields. So, your best bet is just to become a regular to this site.
The best thing is, the second step is completely optional.
I fully agree this feature should be supported. Not only because of English, but also Spanish, French, etc. stackexchange sites. When you read an answer, you should know whether that answer is from the native speaker or not; one should have some measure how the answer is reliable.
I think only one multiple field for Native or Bilingual language proficiency (inspired by LinkedIn) is needed. A lot of people have more than one primary language (Canadians, Belgians, Swiss, people from bilingual families etc.).
On the other hand, I don't think secondary language field would be necessary for this purpose. What does it tell you about the knowledge of the answerer? Not much, I guess. 99% of people who visit the site have that particular language as a secondary language. So, let's make the request as simple as possible, so that it has chance to be implemented!