Some component language packs cannot be installed in Extension Manager (e.g. because they don't include XML setup file). In such case, they need to be installed manually.
See the following example (German language pack for Phoca Gallery component), how to install language pack manually:
Download German language ZIP pack and unzip it. You should get two files:
Copy both files into the following destination:
- administrator/language/de-DE/ folder and
- language/de-DE/ folder on your server.
If the German language is active (admin, front) on your Joomla! site, then the Phoca Gallery component will be translated to German.