I want to really echo the allow as much time as possible part. It's been taking me even longer than normal recently to get through security so that even when there's no line, it can take me 20-30 minutes (I use a wheelchair and need to get patted down). Many airports don't seem to be the most organized about making sure there are people around who can do pat downs.
Metal detectors use magnets so your father's clip would set those off (that's how they detect). He (or your mother) should tell the screeners that he has the clip and that it will set the scanner off. The metal detector wouldn't be unsafe for him like an MRI is, but they'd end up doing a body scan or pat down anyway. They'll either send him through the body scanners (which do not use magnets) or do a pat down. As far as the language issues, he or your mother should tell TSA so they know, but hopefully it won't be a problem!
One thing to make clear to him and your mother is that if he does need a pat down, your mother can watch and talk to him, but she cannot touch him or give him any of the bags or anything until he's been cleared. My father didn't know that last time I flew with him and he tried to hand me my shoes while I was still being screened (I usually fly solo and I don't think he'd ever flown with me without my mother also there before). I always get my pat down done right by TSA, but I think if he prefers a private screening, she can still watch.
They might benefit from using airport assistance to get around the airports. That will give them another person to assist with security and may get them taken through a separate line rather than having to worry about understanding the different types of lines (if the airport uses them; my local airport doesn't). I don't know how big their home airport or Sanford are, but it may also conserve some energy for your father and/or mother to get assistance and be pushed through in a wheelchair.
