I wouldn’t recommend hiding them, especially if you leave the setting to include HTML detection on. Power users may still want to have their (sanitized) HTML parsed as such, not formatted as code. Two examples where this can be useful are kbd and abbr tags.
If you exclude HTML tags from detection (which may be viable depending on the scope of your forum), hiding the “don’t show this again” would probably be OK. I still wouldn’t recommend hiding the “post anyway”, though, because there are bound to still be some edge cases of false positives (I hit one the other day where I’d omitted a space before an opening parenthesis — poor typesetting, but not unformatted code). Then you’ll have a situation where users can’t post their question at all, and, unless they report the issue to you directly, you won’t even know about it.
Language detection is beyond the scope of this component, I’m afraid. Where possible, it looks for syntactical features shared by many languages, such as lines ending in semicolons, certain configurations of curly braces, and so on.
I am thinking about ways to enhance the UX, though. One big improvement would be to make it much more interactive. For example, instead of a simple modal, the user would be presented with a wizard that first asks them which language their post concerns (select from a dropdown), then a screen which asks them to select which ranges of their post are code (defaulting to lines that contain strings flagged by the plugin), then generating the appropriate markdown. This would still include a “skip and post anyway” option, though, for the reasons I mentioned.
I don’t have a timeline for this change, but it’d be good to know if it’s something people would be interested in.