Starting with English, then Japanese and finally for French, I have been using vocabulary apps for almost 8 years. Here's a review of one ofthe most popular apps I have used. I write about why some work for me, and why others don't. When they don't, I interview fellow users.
Duolingo

Who is it for? 👤
• You are just starting a language
• You're extremely dedicated and self-disciplined to progress over time here
• You are easily influenced by gamification and the social aspect of learning
Price 💸
Free, with premium options.
First off, Duolingo is the go-to solution for people who want to start exploring a new language. In other words, it is targeted for beginners. Their designs rely heavily on gamification, in both features and aesthetics, to keep users' energy level high. At times, I personally find it too childish and flashy; the gamification distract me from meaningful comprehension.
Nonetheless, I see a lot of value in using Duolingo early on. I have a friend who is a loyal user and who will keep sharing his progress and badges to stay motivated. With his dedication to learning along with his weekly speaking sessions on other platforms, he is already able to hold an hour-long conversation in Spanish.
Where Duolingo lacks potential is when you consider its use as part of the larger learning experience. Once you have past an absolute beginner's level, Duolingo feels slow when you want to progress faster and more seriously. At that point I believe you need to switch to more intense learning methods with a focus on vocabulary, grammar, listening and speaking.
However, I've noticed it is hard to balance my time on Duolingo with the rest of my learning process. For example, I like to learn grammar from a book in order to have a structured understanding of the language system in my head. But when I go back to Duolingo, I feel behind compared to my personal progress, as a result of all the other exercises I did outside the app.
I believe Duolingo is good for highly self-motivated people starting a language.
But that's not for me, a lazy learner trying to strengthen my English.
Lockcard

Who is it for? 👤
• You like to look up new words in a dictionary
• You often forget words after you search them
• You're a lazy learner who spends too much time on your phone
Price 💸
Free
Lockcard is an improved version of Vocabulary.com with additional features that make it a great app for lazy people. It's an improved English dictionary that uses 'spaced repetition' via notifications to make you remember words forever. It doesn't teach you new words (yet), but it's a great tool to assist you with the rest of your learning process.
It's worth noting that Lockcard is a young app built by two language enthusiasts who happen to work as product designers by day. Therefore, there is a strong focus on smart interactions and minimalist visual design to give you a comforting experience every time you open the app. However, they currently lack in some resources; their dictionary is not as complete as Vocabulary.com
I use Lockcard to search new words I encounter in daily life and when reading. Before, I used to search new words in a traditional dictionary and forget them the next week. With Lockcard, I access only the information necessary in that moment and go back to what I was doing. Throughout my day, Lockcard will remind me that word by sending notifications to my phone, acting as flashcards for my brain. I feel relieved knowing I will not forget the word I have searched. In fact, it makes me more confident in searching words up.
That's the one. Try it yourself for free on the AppStore. Plus, you'll be supporting two awesome language enthusiasts.