Fox Contact includes all available localizations inside its main installation package, but not all of them are installed.
During installation of any extension, Joomla automatically discards localizations related to languages different from the language packs currently installed.
In other words, if you have only English Joomla language pack, during Fox Contact installation, only English localization for Fox Contact will be installed.
After that, if you install another Joomla language pack, Fox Contact doesn't have a corresponding localization files, because they have been discarded during the first installation.
There are two ways to fix this, depending if you can or can not overwrite Fox Contact source files.
If you didn't make changes to Fox Contact source files, install Fox Contact again without uninstalling it first.
During this installation process, new Joomla language packs will be detected, and the corresponding Fox Contact localization files will be installed as well.
All Fox Contact's source files will be overwritten with the version taken from the installation package, but your configuration and all your forms will be kept untouched.
Otherwise, if you made some changes to Fox Contact source files, a subsequent Fox Contact installation would overwrite your modifications.
So extract language files from the installer package and upload them in the corresponding directory.
Example: Suppose you want to manually install Italian localization for Fox Contact.
For the admin side, search into the Fox Contact installer package for the following files: