Just a quick blog post: I regularly have to reinstall a language pack in Sitecore, but often can’t find the official instructions from Sitecore.
Language packs are dependent on your Sitecore version, but not all Sitecore versions have a new version of a language pack. The Sitecore version-independent instructions can be found at Sitecore Experience Platform Client Translations Manual. This page has a warning about installing newer language packs in newer Sitecore versions, but not vice versa!
For example, the newest Dutch language pack was published with Sitecore 8.0-update 4. When installing Sitecore 8.1-update 3, we’ll use this language pack.