The manual is wrong/misleading. I e-mailed the maintainer and they - correctly - notified me that the class sits right inside the gettext repository, at gettext-runtime/intl-csharp/intl.cs.

Currently it does not seem to be part of any pre-compiled library, but it is small and it works as described, so one may just include it directly and build it as part of the consuming project, similar to e.g. the SQLite Amalgamation or the Boost Header-Only Libraries.