Your assessment is largely correct.
For multiple language support, I assume the main issue is labelling of sliders etc? One option is to have a slider to select the language. This would work best if you had the UI in the graphics area rather than using the default sliders. This requires a lot more coding, but it would give you the option of selecting the label text to suit the language. From reaper version 6.30, it is supposed to be possible to show and hide the default sliders, so you could have multiple versions of default sliders, which you would toggle on or off depending on the language. However, I have tried the show/hide slider thing and, to be honest, I couldn't get it to work
The other issue is that you are limited to 64 sliders, which may limit the number of languages you could support. If you create your own sliders in the graphics area, you can have as many sliders or language options as you like.