I solved this problem the following way:
1. Uploaded ru.lng and ru.t9 into tpa/preset/system/language folder
2. Created the file customize_upgrade.xml and put in it the lines:
<?xml version="1.0" encoding="UTF-8"?>
<customization>
<info>
<allowed-language>ru</allowed-language>
</info>
</customization>

3. Uploaded this file into tpa/preset/custom
4. In the "Menu>Settings>Language>Writing language" chose Russian