# EPUBCheck OPF-092: malformed language tag, how to fix it

Source: https://epubcc.com/epubcheck/opf-092/ (updated 2026-11-04)

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. EPUBCheck quotes the tag and the reason; reading apps use the language for hyphenation, fonts and text-to-speech.

- 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".
- Underscores instead of hyphens, such as "en_US".
- Typos or extra spaces in dc:language, xml:lang or lang attributes.

## How to fix it

1. Read the tag EPUBCheck quotes and find where it is used.
2. Replace it with a BCP 47 code: en, en-US, es, pt-BR, ja, ar.
3. Use the same code in dc:language and in the lang and xml:lang attributes of the content.

## When it's not this

If dc:language is missing entirely, EPUBCheck reports a schema error (RSC-005) instead.

## Sources

- EPUB 3.3, §5.5.3.1.3 The dc:language element: https://www.w3.org/TR/epub-33/#sec-opf-dclanguage (read 2026-11-04)
- EPUB 3.3, §5.3.7 The xml:lang attribute: https://www.w3.org/TR/epub-33/#attrdef-xml-lang (read 2026-11-04)

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