Localization Client allow me only to translate from English to one of others languages.
It is possible to translate from language x to English too?
The reason is the way the Drupal translation system is built. Original strings (defined in modules) must be written in English. This means all translation will be from English to X-language.
The Locale module (in core) will detect your t() strings and provide an interface for translations. Handbook page explaining it in summary can be found here: Translating a site interface to different languages (Locale core module).