What causes "{lang} is not a valid language." in langgenius/dify?

Pass one of the supported locale codes (e.g. 'en-US', 'zh-Hans').

langgenius/dify · error · ValueError

{lang} is not a valid language.

Error message

{lang} is not a valid language.

What it means

Raised by supported_language(lang) when lang is not a key in language_timezone_mapping. The supported set is the 23 locale codes in api/constants/languages.py (en-US, zh-Hans, zh-Hant, pt-BR, es-ES, fr-FR, de-DE, ja-JP, ko-KR, ru-RU, it-IT, uk-UA, vi-VN, ro-RO, pl-PL, hi-IN, tr-TR, fa-IR, sl-SI, th-TH, id-ID, ar-TN, nl-NL).

Source

Thrown at api/constants/languages.py:35

    "hi-IN": "Asia/Kolkata",
    "tr-TR": "Europe/Istanbul",
    "fa-IR": "Asia/Tehran",
    "sl-SI": "Europe/Ljubljana",
    "th-TH": "Asia/Bangkok",
    "id-ID": "Asia/Jakarta",
    "ar-TN": "Africa/Tunis",
    "nl-NL": "Europe/Amsterdam",
}

languages = list(language_timezone_mapping.keys())


def supported_language(lang):
    if lang in languages:
        return lang

    error = f"{lang} is not a valid language."
    raise ValueError(error)


def get_valid_language(lang: str | None) -> str:
    if lang and lang in languages:
        return lang
    return languages[0]

View on GitHub (pinned to ef8544b173)

Solutions

Pass one of the supported locale codes (e.g. 'en-US', 'zh-Hans').
Map bare codes to a supported default, or use get_valid_language which falls back instead of raising.
Normalize underscores to hyphens and append a region if missing.

Example fix

// before
supported_language('en')
// after
from constants.languages import get_valid_language
lang = get_valid_language('en')  # falls back to en-US
// or pass an exact supported code:
supported_language('en-US')

Defensive patterns

Strategy: validation

Validate before calling

from constants.languages import languages

def is_supported_language(lang: str | None) -> bool:
    return lang in languages

Type guard

from constants.languages import languages

def is_supported_language(lang: str | None) -> bool:
    return isinstance(lang, str) and lang in languages

Prevention

Prefer get_valid_language for untrusted input — it falls back instead of raising.
Normalize browser locale codes (underscore -> hyphen) before calling supported_language.
Keep a whitelist aligned with language_timezone_mapping keys.

When it happens

Trigger: Calling supported_language('en') (missing region), 'en_US' (underscore vs hyphen), or any unsupported locale such as 'de' or 'zh'.

Common situations: Browser locale negotiation yielding a bare language code, an Accept-Language header mapped naively, or hardcoded locale strings that differ from the supported keys.

Related errors

AI-assisted analysis of langgenius/dify@ef8544b173 (2026-08-12). Data as JSON: /api/errors/71c00f61571f28ad. Report an issue: GitHub.