When comparing Bilingua vs Readlang, the Slant community recommends Bilingua for most people. In the question“What are the best language learning apps?” Bilingua is ranked 5th while Readlang is ranked 7th.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Looks really nice
Pro Effective
Pro Efficient
You're not just chatting with native speakers, you get to practice with their bot as well. There's also a vocabulary list that lets you store words from your chats--really helps with learning the language more.
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 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.