Languages
The language is detected automatically. Each calibrated language has its own calibration, because the same writing lands differently in every language.
Calibrated
English, Spanish, French, German, Portuguese, Italian, Dutch, Polish, Russian, Ukrainian, Turkish, Arabic, Hindi, Chinese, Japanese, Korean, Bulgarian, Catalan, Czech, Croatian, Slovak, Slovenian, Greek, Indonesian, Romanian and Vietnamese.
Other languages
Texts in other languages still get a score, with language_supported: false and confidence: "low", so they always come back inconclusive.