Right..I think you're touching a good question here. The Opencpn
plugin download page offers several compiled files for respective operating system. For Windows a .dll file. But installation
of corresponding .mo files are not there except for Dashboard_pi and Grib_pi which are include in the OpenCPN installation
process. That implies for user not familiar with source code and similar they wont get use of the translations done.
The best way may be if the plugin publisher also served a routine to automatically copy present .mo files to .......\opencpn\share\locale\....(is this structure the same for all system?)
An other method could be to include the present .mo files in language dependent directories and named as needed for respective operating system. Like opencpn-wmm_pi.mo. And ad a description how to copy the files. Not all of the users can easily perform this but it may be better than nothing?
And of course everything available at opencpn.org.
These are my instant reflections but someone else must have been thinking of this topic before?