Merge branch 'custom_language_translation_vtiger7x' into 'master'

Custom translation for module

Custom translation without overwrite vtiger core files, for all modules defined in languages/custom/$language/$module.php file(s)

1/ You need to create a /custom directory in /languages directory

2/ Then create $language (en_us, de_de, ....) sub-directory

3/ Create a tranlation file like Accounts.php in languages/custom/en_us/ directory and define new tranlation or overwrite existing translations keys.

This small change will keep original Vtiger language file and allow users to upgrade Vtiger version without lossing own translaction.

See merge request !585