Yes, I just now made any kind of effort to teach the actual cue "Heel" (and, no, I don't know a good Spanish word for it, yet!).
You read that correctly. No, this doesn't mean I don't require Wally to walk at heel position. I just never made any sort of verbal or hand signal for it. I just expected him to do it, and taught it using the "be a tree" method, and marking and rewarding him for coming back into position on his own. He would also get marked and rewarded for maintaining position while we're on the move.
The reasoning is that at the time I had just learned about "default behaviors", a behavior the dog will choose first time, every time when presented with the situation. Using sit as the default behavior helped him master sitting before being let out or coming back in, further cementing the idea as a good one.
So I wanted to extend that to heeling. I wouldn't need any cues because it would just be the default, automatic behavior. "We're walking? I get in heel." That's what I wanted him to think. Nothing to cue because he's already doing what I would be telling him to do. When he would wander out of position, I usually just redirect and stop moving (which then had it's own default attached, coming back to my side), though I did use "keep up" for when he's lagging, and "slow down" for when he's going too far ahead, and he picked up on the meanings of these cues by mark/rewarding when he gets back into position.
For the longest, I didn't see the need for a "real" cue for heel. Staying in position is the default. If he gets distracted or something, I can use "keep up"/"slow down". Then for some reason, I just decided to start putting the cue while he's doing it and marking and rewarding (easy, since he's already doing it, I'm just telling him what it's called). When he wanders out of position, I would say "heel". He probably didn't know what it meant (or did he?) but since he always before just come to my side, he'd do it and get rewarded.
I don't know why I just got it in my head to do it, but...there it is.
I guess the real question is: "What's the cue for 'heel' in Spanish?"