From Dolibarr Open Source ERP CRM Wiki
Development rules for internationalization (i18n)
In Dolibarr code, translations are made by using a method available in object $langs with the key to translate as a parameter of this method. The $langs object contains definition of use language. This object is always defined in all pages.
There are two methods:
This method returns the translation of key TRAD_STRING, and encodes the result into HTML entities to allow to display correctly (even with special characters) on an HTML page.
This method returns the translation of key TRAD_STRING, but the result is the translated string with no particular encoding (string is stored in UTF-8 in memory).
These two methods can accept arguments that will be used to replace strings %s in the translated string.
Note: If, as arguments, you use a string that is itself a translated string, you must use method $langs->transnoentitiesnoconv, otherwise the string will be encoded twice. Example:
Translate Dolibarr into another language
See page Translator documentation