Instead of having a switch-case returning a function to be executed, it's
simpler/faster to have a single function containing a switch-case. It also
allows to group languages with identical plural form in a single
implementation, and remove the "default" guard value, as switch-case already
have a `default:` case.
While doing some profiling for #2900, I noticed that
`miniflux.app/v2/internal/locale.LoadCatalogMessages` is responsible for more
than 10% of the consumed memory. As most miniflux instances won't have enough
diverse users to use all the available translations at the same time, it
makes sense to load them on demand.
The overhead is a single function call and a check in a map, per call to
translation-related functions.
