Translate Contact Enhanced Form Validation errors for Joomla 3.0 and newer is a very simple task, however it might be a bit more complicated for Joomla 2.5 users. Please continue reading...
Contact Enhanced 3.0 and newer uses jQuery for the form validation script. The validation messages are in the Contact Enhanced Language files, so what you need to do is translate Contact Enhanced. For more information on how to translate Contact Enhanced read these articles:
[Path to Joomla 1.6+]/media/system/js/mootools-core.js
[Path to Joomla 1.6+]/media/system/js/mootools-more.js
ar-AA, ca-CA, ca-ES, cs-CZ, da-DK, de-DE, en-US, en-UK, en-AU, es-AR, es-ES, et-EE, fa-IR, fi-FI, fr-FR, he-IL, hu-HU, it-IT, ja-JP, nl-NL, nl-BE, no-NO, pl-PL, pt-BR, pt-PT, ru-RU, si-SI, sv-SE, tr-TR, uk-UA, zh-CN, zh-TW
If it is not translated into your language, open this file:
[Path to Joomla 1.6+]/components/com_contactenhanced/assets/js/mootools-languages/locale.en-US.js
and copy to:
[Path to Joomla 1.6+]/components/com_contactenhanced/assets/js/mootools-languages/locale.language-CODE.js where language-CODE follows this convention [language-code]-[COUNTRY-CODE]. (see a list with the most common COUNTRY-CODE)
Then Translate all content of the file inside single quotes, except for the strings 'Date', 'Number' and 'FormValidator'.