One of the biggest struggles when learning French was remembering where to place the accents and learning idioms/euphemisms. Luckily I found the Memrise app, which has drastically improved my French vocabulary! How does it work?
Memrise uses mnemonics and other memory strategies to help create associations between the English language and the French word you are trying to learn. In turn, your likelihood of remembering the word increases.
Various Memrise users will post different strategies on how they learned a specific word. Memrise has courses in almost one hundred languages of all levels. I even learned the Russian alphabet just for fun!
To ensure that the new knowledge you gain sticks in your memory, the app shows a flower in the top right corner of each word. If you have practiced a word enough time, then the seed will grow into a pink flower (see picture below).
Memrise also has other strategies to review vocabulary such as typing out the translation and multiple choice questions.
I would highly recommend Memrise for more difficult languages that are not similar to English such as Mandarin. Because of Memrise, I was able to remember at least 20 different characters and how to write them.
The only drawback of Memrise is that it is not as effective at teaching grammar. Ideally you would use Memrise to learn vocabulary and then you would use another app such as Duolingo or FluentU to learn grammar.