To install a language pack, click on Language Packs on the Tools menu, to open the Language Packs window. If the language pack you are looking for isn't listed there, click the button and choose From Family Historian Plugin Store. This will show a list of available language packs. You can select one and click to import it.
If instead of installing from the Plugin Store, you have a language pack file that you wish to install, do the same thing, but when you click on the button, instead of choosing From Family Historian Plugin Store, choose From File, and select the language pack file.
Family Historian will never allow an imported language pack to replace an existing language pack. If you import a language pack with the same language id as one already installed on your PC, Family Historian will modify the id of the new one, to make it unique. For example, if you have a French language pack installed, with language id "FRA", and you install another French language pack with the same id, Family Historian will modify the newly installed language pack's id to "FRA 2". You can delete unwanted language packs at any time, and also rename them or their ids, as required.
If you have an old language pack and you want to upgrade to the latest version, simply install the latest version from the Plugin Store. This will install the new version alongside your old version. If you no longer want the old version, you can delete it.
If the language pack you want isn't available (or there is one, but it isn't adequate for your purposes), you can create your own - see How to Create a Language Pack.