It is a common misconception that language and speech develop together. In reality, they are two very different things.

Speech is the physical act of talking.

Language is the system of symbols and rules we use to string our thoughts together - both expressively and receptively (and written, but that's another whole different conversation.)

While they are closely related, they develop separately.

RED FLAGS: 
by 18 months: has trouble imitating sounds. has trouble understanding simple verbal requests. 
by 2 years: can only imitate speech or actions and doesn't produce words or phrases spontaneously. 
by 2 years: says only some sounds or words repeatedly and can't use independently and with purpose