To speak easy sentences in your own language is key to speaking other languages.

 I would rearrange this sentence to read, "The key to speaking other languages is speaking in easy sentences in your own language." 

If you want to speak in English, ("in" is OK here, but it is unnecessary) you need to make your countries country's language simple in the first place. Because If you use simple words, you could can change it to English easily.

In Japanese, for example, people tend to use complicated sentences and difficult words as a matured communications. I'm not sure what you mean at the end of the sentence.  Maybe you mean "in order to demonstrate fluency?"

But, the more complicated it is, the more difficult it will be to speak say in English. The way to solve these issues, you should is to use your simple sentences in your mother language as a habit.

よくできました

The key to speaking other languages is speaking in easy sentences in your own language.

If you want to speak English, you need to make your country's language simple in the first place. If you use simple words, you can change it to English easily.
In Japanese, for example, people tend to use complicated sentences and difficult words as a sophisticated communications. But, the more complicated it is, the more difficult it will be to say in English. The way to solve these issues is to use simple sentences in your mother language as a habit.