When should strangers be able to understand my child's speech?

Comes in stages: Most kids will emerge as copycat talkers, trying to mimic what they hear. Clarity comes with repitition, gental redirection and eventual confidence. About half what they say at 2 should be understood by strangers(3/4 by 3 and 95% by 4). Any kid that doesn't should be screened. School districts operate child find programs to get these caught up by school entry.

Speech: By 3 years old. Children will still have speech immaturity, but you shouldn't have to 'translate'. Take your child to the pediatrician, and see if they think s/he needs an evaluation by a speech therapist. Earlier intervention makes treatment much easier!