If a language you operate in is not available as standard, then you can create your own translation. Here's how:
You will find the language file in this directory modules > addons > ADDON_NAME > lang
Begin by opening an existing language file
Save this file with a new name - the name should be the same name of the language file in your admin language folder in WHMCS - the name should only consist of a-z0-9 characters and end with the extension ".php"
Once the file has been created, you can then begin going through and translating the lines within it
An example line is as follows - you should only change the part in bold:
$_ADDONLANG['addonname'] = "Addon Name";
Be careful not to delete any of the quotation marks (") around the text strings or the semi-colons on the ends of each line (;). Also should you want to use a quote character (") within your translated text, you must escape it - for example: \" The language files are written in PHP syntax so valid PHP code must be maintained.