Your IP Office system administrator can configure which languages are available for use by the phone for its menus. You can select which of those languages the phone uses.
The IP Office also sends text that is displayed in some of the menus. Wherever possible, the IP Office and the phone try to use the same languages. For example, if you change the phone language, the IP Office will try to send text to the phone in that language and to change your voicemail to use that language for spoken prompts.