ISPConfig 3 has a builtin language editor to easily create translations of the ISPConfig interface.
Creating a new language file set
Login into ISPConfig as admin user. Click on "System" in the upper menu and then on "New Language" in the left menu. As language basis you should always use "en" as this is the master language file set in ISPConfig. In the field "new language" enter the iso code for the language that you want to create, e.g. "de" for german or "fr" for french. The iso code shall be lower case. Then click on the "Create new language file set" button at the bottom of the page.
List of all ISO language codes: http://en.wikipedia.org/wiki/List_of_ISO_639-1_codes
Translating the languge files
Login into ISPConfig as admin user. Click on "System" in the upper menu and then on "Languages" in the left menu. Then select the language that you want to alter in the "Select Language" option field and start the translation by clicking on the langauge file name below and editing the text strings in the form fields. to save the modifications, click on the "Save" button.
Exporting a language file set
The Language editor also contains a function to export language file sets. This is useful if you want to make a copy of your custom language file set before you update ISPConfig or if you like to send a language file set to the ISPConfig developers to be merged into the next release.
To export a language file set, login into ISPConfig as admin user, click on "System" in the upper menu and then on "Languages" > "Export" in the left menu. Select the language that shall be exported and download the language file.
If you like to contribute your translation to the ispconfig project, please send the exported language file by email to dev [at] ispconfig [dot] org