"... and are satisfactory for this case." ie. They fulfill all of the requirements for this particular type of Mexican document.(whatever those requirements might be but not necessarily referring only to the mental capacity/competency of the witness.) Mexican law, for example, (and only by way of example) might stipulate a minimum requirement for witnessing this type of document:
1. Must be two witnesses
2. Witness must live in the area
3. No convicted felon may witness
4. No formerly evicted tenant may witness
5. etc.

So you require a witness that is "adequate for the case". Don't get hung up on "mental capacity". As long as the conditions of the statute are met then the witnesses are "sufficient".

That's how I see it, María. ¡Suerte! (as they say)