I'm a native English speaker, and I couldn't find anything in the tour or the help center about this question. Is it appropriate for me to answer questions on this SE? I likely will never ask a question here (my questions would go to English Language and Usage instead). I know that this SE is geared more towards people trying to learn or master English, but is my input appropriate here? I'm interested in sharing my language (and culture) with those interested in learning it.
Absolutely your participation is welcome here, in any role.
I suspect, in fact, that most learners ask questions here precisely because they want “authoritative” answers they believe only native speakers can provide.
(True, they quickly discover that a) there are no authoritative answers; b) non-native speakers often provide better or more relevant answers; and c) native speakers agree with each other about as often as clocks do. But these are all important things to learn, too.)
One of the real strengths of this site is the variety of answerers. The majority are Anglophones of one sort or another; but many of them speak decidedly non-standard dialects like East Alabama and Scots, and we also have very highly regarded answerers whose native languages are Thai, Dutch, Hindi, German, Russian, Italian ... you name it, we've probably got it.
So please, jump in. But be warned: the questions here are often much harder than those on ELU. You are likely to end up learning more from the questions than you can possibly return to the questioners.
We get quite a few questions on ELL asking which is "correct" out of two or more alternatives. Often, practically every native speaker could easily identify the most common (or only "correct") usage. So like many others, I'm often tempted to post a short answer addressing the "bare" question as asked, but...
Adapting for context, whilst all "correct" answers are welcome, by far the most valuable ones are those that illustrate some relatively reliable principle about English usage that the OP can apply to a wider range of contexts than simply ticking the right answer in a multi-choice homework test.
TL;DR: Being a native speaker makes it easy to provide a basic answer to many questions here, but please consider what else you might be able to add that will extend the scope of whatever you say.