This post is relevant to all Cicso SPA phones, which have “G” letter in their names. “G” means global (that you can put there languages), not gigabit ethernet (all phones have 100 Mbps interfaces).
First of all, you need to download language files from cisco support page. They are XML files and look like “spa50x_30x_en_v756.xml”. This is for english language. Important is, that you must have two files – for your desired language and for english language. Then set up TFTP server and put there files.
Log in to the phone via web browser and on “Regional” page on the bottom, there is box named “Dictionary Server Script:”. Then put there
serv=tftp://192.168.1.1/;d0=English;x0=spa50x_30x_en_v756.xml;d1=Slovak;x1=spa50x_30x_sk_v756.xml
It is very important, that english file is first, and then there is your desired language (we have slovak language there). Submit changes, and then in your phone under option 17 – Language, you can change language.