None of the “language learning apps” will effectively help you learn a new language to the extent of being comfortable using it in practice.
I’ve studied Spanish and German with Duolingo, Memrise, Babbel and enjoyed different aspects of each app. They weren’t mutually exclusive, either.
Most applications will teach you some basic grammar and a limited set of words and phrases. Some (like Babbel) include basic conversations in their curriculum which is better than nothing.
Unless you supplement your learning with additional reading, watching TV in the foreign language (with subtitles), trying to listen to radio or music, and communicate with native speakers, you’ll be caught off-guard the first time you travel abroad or have a business meeting.
In a natural conversation, you normally have metaphors, idioms, slang, cultural references — tons of context that isn’t taught with apps. Dialects and accents make a huge difference as well. Studying purely from a “textbook” (especially if you skip the voice exercises) will limit you further.
Some training programs require video or audio conference conversations with other students. This actually makes a bit more sense and can supplement the learning.
Note: None of the popular language learning apps is poor per se. They simply don’t help you participate in an actual conversation at the restaurant, at the bus station, in the cab.
You tend to cover a dozen basic phrases which never come up in practice, and get stuck desperately trying to match what you hear to the limited list of phrases you’re prepared to translate.