[zhennai]

It's almost same meaning.
I think "言葉" has broader meaning. If you define "言葉" is a system of sounds and words to communication used by human, "言語" is the system of communication in speech and writing that is used by "particular groups of people". (Strictly speaking, 言語 has both meanings, but we prefer using the latter meaning.)

Ex,
1. spoken language→話し"言葉" not "話し言語"

2. Which language should I learn?→どの"言語"を学ぶべきか？
or
I know the language of a country. →その国の"言葉"を知っている。

My English is not so good, hope you can understand it...