Zotero automatically uses the language of the Firefox version you are using.

If you'd prefer Zotero to be in a different language from Firefox itself, type about:config into the Firefox address bar, look for general.useragent.locale, double-click it, and enter an appropriate locale code. For example, to use English, set it to en-US. See the chrome manifest for other available codes. This change will affect all localized Firefox extensions as well as websites that auto-detect visitors' languages.

You must restart Firefox for the change to take effect.