When comparing Lang-8 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 Lang-8 is ranked 7th. 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
Ranked in these QuestionsQuestion Ranking
Pros
Pro Gain experience writing in your target language
Where many programs will have you writing single sentences at most, Lang-8 encourages you to write short stories or diary entries.
Having to actually write in the language rather than simply remember vocabulary is a really beneficial feature for improving real-world use of the language.
Pro Get feedback from native speakers
Native speakers of the language you are learning will give you pointers on your writing, such as grammar corrections and advice on word choice/sentence structure.
Pro Self-directed learning
Write as much or little as you like, about the subject of your choice, at the level of difficulty of your choice, taking as much time as you wish. Intermediate and advanced learners can benefit the most from this, but beginners can still use the site to practice what they've learned elsewhere.
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 New sign ups are currently put on hold
They are not accepting new registrations now, and it is not clear when will they accept them again.
Con Not a good starting place for beginners
The site doesn't teach you your language, instead it allows you to get advice on your writing in your target language. It's better if you have a basic understanding of the language at first.
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.