In order to install new translation you have to download language you want first and extract somewhere temporary files. Archive contains two folders:

1. dc - holds translations files for desktop client
2. wc - holds translations files for web client.

Installing translations for web client.

If you want to install translations for web client you have to enter "wc" folder and copy internal folder, example "lt_LT" to web application "translations" folder.
Then you have to edit "settings/settings.ini.php" file. If previously file part looked like:

'available_locales' =>
array (
0 => 'en_EN',
),

'available_locales' =>
array (
0 => 'en_EN',
1 => 'lt_LT',
),

If you want to change dafault language you have to edit "'locale' => 'en_EN'" and replace with your preferred language.

Installing translations for desktop client.

Now you have to enter "dc" folder and copy two files to desktop client "translations" folder. Then you have to edit "settings.xml" file and edit "<language>en</language>" variable.

Thats all :)