Fluency in speech is not the ability to know every word and grammatical formation in a language; it’s the ability to use whatever words and grammar you know to say whatever’s on your mind. When you go to a pharmacy and ask for “That thing you swallow to make your head not have so much pain,” or “The medicine that makes my nose stop dripping water” – THAT is fluency. As soon as you can deftly dance around the words you don’t know, you are effectively fluent in your target language. This turns out to be a learned skill, and you practice it in only one situation: When you try to say something, you don’t know the words to say it, and you force yourself to say it in your target language anyways. If you want to build fluency as efficiently as possible, put yourself in situations that are challenging, situations in which you don’t know the words you need. And every time that happens, stay in your target language no matter what.
speaking fluency comes from staying in the target language.