When comparing Language Zen vs Readlang, the Slant community recommends Readlang for most people. In the question“What are the best sites for learning foreign languages?” Readlang is ranked 4th while Language Zen is ranked 38th. The most important reason people chose Readlang is:
You can read any content in your target language using the browser extension, and translate any words/sentences you don't know quickly.
Specs
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Pros
Pro Translates native content quickly
You can read any content in your target language using the browser extension, and translate any words/sentences you don't know quickly.
Pro Creates flashcards from words you don't know
Readlang takes the words you translate most often and turns them into flashcards for you to practice.
Pro You can import long texts, including whole books
This is impossible or very annoying with other similar services.
Cons
Con Limited use
So far the only language available is Spanish. While more may be planned, currently this is only useful to those that want to learn Spanish.
Con Not a good way to learn a language by itself
It's not your standard learning website with structured courses. Instead, it helps you get more familiar with your target language by translating content when needed.
This can be really beneficial for someone trying to improve their fluency, but isn't going to directly teach you grammar. Readlang is better used to supplement your learning while learning from structured courses.