Guess I just leave those long lines alone manually wrap inside a region

Maybe I misunderstood. Fold/nest (which I was talking about) implies the hiding of lines from a block structure, such as a c-function, like

⊟ int blah(args) {
    int a = 1;
    int b = 2;
  }
⊟ double yo(args) {
    double a = 1;
    double b = 2;
  }

being collapsed down to

⊞ int blah(args) {
⊞ double yo(args) {

Whereas “long lines … wrap” is a completely different setting (View > Word Wrap or equivalent toolbar icon), and just converts

13    Pretend this line... is very, very long
14    And another line

into

13    Pretend this line...
      is very, very long 
      (but now it is wrapped)
14    And another line

Is this possible using UDL? is there a plug-in that would do this?

As far as I know, UDL will not fold/nest based on indents, only on character sequences. Though if someone can prove me wrong, great!

It is possible to write a plugin to do that – because it’s theoretically possible to write your own lexer plugin. But I don’t know of any that already exist.