Translation API with free tier and reasonable prices.
Lecto Translation API is a free Text Analysis API that requires apiKey authentication, is served over HTTPS and supports CORS, so it can be called from the browser.

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Free text translation with language detection, supports 50+ languages
Translate between 21 of most used languages, accurate, unlimited requests
Multilingual sentiment analysis of texts from different sources
Yes. The Lecto Translation API is listed in the Text Analysis category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
Yes. The Lecto Translation API uses apiKey authentication, so you need to register for credentials before making requests.
Yes. The Lecto Translation API supports CORS, so front-end JavaScript can call it directly.
Yes. The Lecto Translation API is served over HTTPS, so it's safe to use in production.