In one of my new Umbraco 8 sites, I needed to create a dictionary to store texts, rather than hard-coding strings in C# code.
The solution is to login as administrator, and assign the user access to the group Translators, or to enable the section Translators for the group.
It seems that an administrator does not have access to the translation functionality by default.
Then, create Languages under the Content tab, and add dictionary items with their respective text values under Translation.
See the Umbraco forum for illustrations.