# EPUBCheck OPF-092: Language tag … is not well-formed: …

Source: https://epublab.com/epubcheck/opf-092/ (updated 2026-11-03)

EPUBCheck OPF-092 is an error message: “Language tag "…" is not well-formed: ….” A language tag in the EPUB is not a well-formed BCP 47 tag.

- Official message (EPUBCheck 5.4.0): Language tag "%1$s" is not well-formed: %2$s
- Default severity: Error

## Typical causes

- Language names instead of codes, such as "English", or underscores, such as "en_US".

## How to fix it

- Use a BCP 47 code: en, en-US, es, pt-BR, ja, ar. The language drives hyphenation and text-to-speech in reading apps.

Message text and severity: EPUBCheck (W3C, BSD-3-Clause). Explanation: EPUBLab.