The following locale directories hierarchy is used in Plesk:

 For Plesk versions lower than 7.5.0:

./htdocs
./htdocs/locale
./htdocs/locale/<LocaleID>
./htdocs/locale/<LocaleID>/help
./htdocs/locale/<LocaleID>/help/cl
./htdocs/locale/<LocaleID>/help/ml_usr
./htdocs/locale/<LocaleID>/help/dl_usr
./plib
./plib/conhelp_<LocaleID>.php3
./plib/messages_<LocaleID>.php3

Each locale is identified by the full name <LocaleID>-<Language>, for example:
ru-Russian
en-English
de-German
...

<LocaleID> - contains the locale identifier (ru, en, de, ...)

The htdocs/locale/<LocaleID> directory contains the help pages.

The plib/locale/<LocaleID> directory contains several files:

• conhelp_<LocaleID>.php - context help messages (small phrases which appear in the control panel left frame when you hover your mouse over control elements);
• messages_<LocaleID>.php - contains localized messages displayed in the control panel;

After the files are installed, you need to register the LP in the Plesk database:  

CREATE TABLE locales (
  id VARCHAR(3)   PRIMARY KEY      -- lang ID: "en", "it", etc
 ,lang VARCHAR(30)                      -- lang name "English", "Japanese", etc
) TYPE=INNODB;

where id=<LocaleID>, lang=<Language>

 For Plesk 7.5.0:

 The files and directories structure:

./htdocs
./htdocs/locale
./htdocs/locale/<LocaleID>
./htdocs/locale/<LocaleID>/help
./htdocs/locale/<LocaleID>/help/cl
./htdocs/locale/<LocaleID>/help/ml_usr
./htdocs/locale/<LocaleID>/help/dl_usr
./plib
./plib/conhelp_<LocaleID>.php3
./plib/messages_<LocaleID>.php3

Each locale is identified by the full name <LocaleID>:<Language>:<LanguageGroup>:<Country>. See IETF RFC 1766 for more details. 

For example:

ru-RU:RUSSIAN:SLAVIC:RUSSIAN FEDERATION en-US:ENGLISH:GERMANIC:UNITED STATES de-DE:GERMAN:GERMANIC:GERMANY ...

Only <LocaleID> is used in Plesk.

Also, the structure of Plesk database for registering installed locales has been changed: 

CREATE TABLE locales (
  id VARCHAR(17) PRIMARY KEY
 ,active enum('false', 'true') NOT NULL DEFAULT 'true'
) TYPE=INNODB;

id = <LocaleID>;
active - allows for selecting the list of active languages, i.e. the list of languages that are going to be used in the control panel. This is necessary in case the number of installed LPs exceeds the limit of the key;

Note: later in this document the <LocaleID> format will be used for Plesk 7.5.0, for example "en-US".