Very important thing is, that your IP phone has to have letter "G" in its name. For example "Cisco Unified IP Phone 7970G" or "7960G". Letter "G" means, that phone is "global", and also support other laguages than english.
Do not confuse it with "GE". This means "giabit ethernet". If phone support both, there is "G-GE" on the phone.
First, download language files for your phone from Cisco support page. You will download .cop.sgn file. Decompress it into .tar file. You can use these instructions.
In the tar file, there are two folders: usr and var. Focus on usr folder. Go to usr/local/cm/tftp. There, you can find folder with your language files. In our case there is "slovak_slovakia".
Copy this folder into root of your TFTP server. All other files in .tar file are for Call Manager only and not interesting for us.
Last thing you need to do, is change .cnf.xml file of your phone. In our case, it can be:
<userLocale> <name>slovak_slovakia</name> <uid></uid> <langCode>sk_SK</langCode> <version>10.2.2.1000-1</version> <winCharSet>iso-8859-1</winCharSet> </userLocale>
Word in "name" tag is name of folder in your TFTP server you just copied.
That's it! Just reboot your phone and enjoy your new language.