Hey Cam
I would suggest you to NOT USE AN APP to learn English. I talk from experience. I know how to speak 6 languages, and from what I have learned is that there is no app that will help you learn a language.I have never used an app to learn any new language. I am not saying that apps are not useful, but in your case you know how to speak English and want to become better.
Duolingo or any other app will help you improve your WRITING and LECTURE skills, but not your TALKING skills. You might be thinking how. The reason is simple. When you use Duolingo or an app to improve your language you will talk that language in your mind . For example :
1) Comment dire : - Je suis un garcon
Answer : - I am a boy
Now you said I am a boy in your head and simply wrote it down. Now this is the problem :- If your friend asks you "Comment dire <<Je suis un garcon en anglais, you will say I am a boy in your head and make sure that there are no mistakes and after that you will tell your friend how to say it. Now the time that you will take will be 30 - 40 seconds. If you want to be fluent and better, than this wont help you answer on spot.
I would suggest that you watch videos and cartoons in English. Watch everything in English with no subtitles for more improvement. Try talking in English more often. If you are trying to talk in English with someone and lets say that you want to say " I went to the supermarket " but you dont know how to say "supermarché". You can try and say " I went to get groceries " or something. You can even ask the person you are talking with " Comment dire supermarché en anglais?" and when they tell you, you wont forget how to say it easily because every time you want to say that word you will remember what the person told you.
You can ask anything else that you would like to know. I am always here to answer. I might not be an AlloProf teacher, but I am a good consultant.