WHMCS includes 26 client area translations by default. These are:
Azerbaijani, Arabic, Catalan, Chinese, Croatian, Czech, Danish, Dutch, English, Estonian, Farsi, French, German, Hebrew, Hungarian, Italian, Macedonian, Norwegian, Brazilian & Native Portuguese, Russian, Spanish, Swedish, Turkish & Ukranian
And for the admin area there are 13 translations included:
Arabic, Czech, Dutch, English, Farsi, French, Hebrew, Hungarian, Italian, Portuguese, Russian, Spanish & Turkish
We rely on the dedication and generosity of our users for contributing translations, as we find real life translations are a much higher quality compared with using automated translation systems as some softwares do. So if you have any suggestions for improvements, or a new language file you are willing to contribute, please get in touch.
Instructions for translating WHMCS into a new language are located at https://developers.whmcs.com/languages/adding-a-language/
In order to remove a language from the WHMCS client area, delete the language file from the /lang directory. To remove a language from the admin area, delete the language file in the /admin/lang directory.
You can also prevent users from being able to change the language of your system by unticking the Enable Language Menu option on the Setup > General Settings > Localisation tab.
Instructions for customising text in an existing language are located at https://developers.whmcs.com/languages/overrides/