Dutch Formal language not available

Hi,

I have several WordPress websites. Some originally downloaded from https://nl.www.ads-software.com/, the nl_NL version, and some orginally downloaden from www.ads-software.com, the en_US version.

I updated both to 4.4.2. For both WordPress installs I want to change the language to Dutch Formal which is now available. However, this Dutch Formal language I only see in the updated en_US install and is not visible in the nl_NL install.

Why is this?

Kind regards,

Willem