Would a mace to the jaw cause a bard or other spellcaster incapable of their vocal components for spells? It seems that I can't make a called shot to the bard's jaw so does this mean I have to hit his body and he can continue to cast? I'm looking for rules as intended and not necessarily RAW since the PHB says that less than half of HP remaining shows as wear such as cuts and bruises but wouldn't a broken jaw be reasonable for a mace to a gnome head.
Dungeons and Dragons is not meant to be an ultra-realistic combat simulator. Targeting specific body parts and getting complicated results based on those is not a feature of D&D 5e.
Adding in this kind of feature empowers certain builds (specifically archer/crossbow builds) in ways that can be very anti-fun. If you can have a group of low level Goblin archers shoot your entire party in the legs to disable them, for example, you're likely to have a very bad time.
In our group we have included optional rules for things like this. However, to sustain this kind of critical damage the character usually needs to be on 0 hp or receive a critical hit. The rationale behind this is that hit points represents a characters ability to defend and not so much his ability to "take hits". So, as long as there are hitpoints to spare the character have effectively avoided any serious hits, and turned potentially fatal and crippling hits into glancing blows or misses, but at the expense of fatigue or accumulating bruises. Once the hps are down to 0 (or otherwise incapacitated), you can in effect do whatever you like with the character and break his jaw or whatever. If the blow is a critical hit, the blow is assumed to land and do proper damage, but not necessarily enough to incapacitate him.
We use a list of random events including things like scars and broken arms or lost eye all the way to instant death. If the character doing the damage cant align the blow properly but does the damage in a fight or something, the damage done is random. If the character can align a blow reasonably well (head blow to a tied down character for instance, the recipient need to be incapacitated) he get to choose.
This sort of thing really brings out the usefulness of spells like Heal or Regenerate since they can be used to repair severed limbs.
If you want to include the random damage due to crits or reduction to 0hp the severity of the list sets a different mood to the game since getting hit can be much more dangerous.
There are no called shots in 5E, your group's mileage may vary but RAW there are none published thus far. It was a bit unfair in other systems unless it was skewed to the players but my opinion is that all combat options should be available to both PCs and NPCs which means that Death Knight's called shots are going to hurt. We call it the Goose and Gander argument at my tables.
That said, there is a section of the DMG Page 272 on Lingering Injuries that could be modified as the ones listed are a bit sparse. These are usually used in game when a crit or massive damage is inflicted to add a bit of grit over the normally and overly esoteric hit point system.
Realistically, two lost hands would prevent Somatic components so a broken jaw would prevent Verbal in all likelihood, of course the former is on that chart whereas the latter is not.