There is a very important relationship between thinking and language. In a sense, they are codependent upon one another. (That is not to say that language is the only way to convey a thought.)
One can easily speak (language) and not convey a message to be thought about. For example, a person may be saying something to another person who is not paying attention. Here, language is being used, but the one who is supposed to be listening and decoding the language (thought) fails to do so.
Here is another example: one must understand what someone is saying (language) in order to think (thinking) about it. Likewise, one must be able to think (thinking) about what he or she wishes to convey and have the ability to convey it (language).
Think about it this way: you are listening to someone speak in a language which you are unfamiliar with. Because of this, you are unable to process the information that person is sharing. You hear the language, yet you are not able to think about what the person is wishing to convey. You can think, yet you are unable to say anything back (perhaps other than you do not understand).
You can also think about it in this way: In a classroom, a teacher speaks to you about a topic. You take what they have given you (language) and process it for understanding and mastery (thinking). If you do not understand what is being said, you cannot process.