If you just want to create a new language for an existing mandator, you need to process step 3 only.
Don't forget to change the database name used for this example: 'idoit_new_mandator'.

Step 1. - Create a new database

Get a connection to your database:

mysql -u root -p

Create your new mandator database:

create database idoit_new_mandator;

Step 2. - Import a dump of the i-doit data tables into your new database.

Switch to your idoit directory:

cd idoit-path/

Insert a fresh database (change idoit_new_mandator to the name you used for your new database in step one):

mysql -uroot -p -Didoit_new_mandator < setup/sql/idoit_data.sql

Step 3. - Insert a new entry into your idoit system database

Get a connection to your database management system and select your i-doit system database:

mysql -u root -p -Didoit_system_database

Modify the quoted fields after the keyword VALUES before inserting, but dont remove these quotes.

Insert a new mandator entry with english language:

INSERT INTO `isys_mandator` ( `isys_mandator__id` , `isys_mandator__title` , `isys_mandator__description` , `isys_mandator__dir_cache` , `isys_mandator__dir_tpl` , `isys_mandator__db_host` , `isys_mandator__db_port` , `isys_mandator__db_name` , `isys_mandator__db_user` , `isys_mandator__db_pass` , `isys_mandator__sort` , `isys_mandator__default_lang_const` , `isys_mandator__default_language_short` )
VALUES (DEFAULT, 'MandantTitle', NULL , 'cache_mandanttitle', 'default', 'db_ip', '3306', 'idoit_new_mandator', 'db_user', 'db_pass', '50', 'ISYS_LANGUAGE_ENGLISH', 'de');

Insert a new mandator entry with german language:

INSERT INTO `isys_mandator` ( `isys_mandator__id` , `isys_mandator__title` , `isys_mandator__description` , `isys_mandator__dir_cache` , `isys_mandator__dir_tpl` , `isys_mandator__db_host` , `isys_mandator__db_port` , `isys_mandator__db_name` , `isys_mandator__db_user` , `isys_mandator__db_pass` , `isys_mandator__sort` , `isys_mandator__default_lang_const` , `isys_mandator__default_language_short` )
VALUES (DEFAULT, 'MandantTitle', NULL , 'cache_mandanttitle', 'default', 'db_ip', '3306', 'idoit_new_mandator', 'db_user', 'db_pass', '50', 'ISYS_LANGUAGE_GERMAN', 'de');

- You can also use another frontend than the mysql-client if you like.