I voted yes of course, but as @mynetx said, this has been done before - not just in Plus! scripts either, there’s been separate applications, P4 (Activities) apps, and pretty much any other way Messenger has been made extendable over its lifetime.
Having tried most (all?) of these at the time, and as a mostly exclusive anglophone, I never really found this concept fully workable in real-world conditions. I’m not sure if it was having to start the application manually beforehand, or the translation services at the time being unable to handle non-formal text very well, I always ended up thinking it’s a great idea but doesn’t work for me. At the very least this should be better now as the translation services will have improved since.
What I’ve used for many years now that does work for me is QTranslate. With that you can select text anywhere (including in Messenger), hit Ctrl twice to translate (and other keys you can setup for things like a dictionary or spell check), and you can click the tabs to have the text translated in other services if your default doesn’t give a good result.
Finally, not to get too far off track in this wall of text, but has anyone tried the built-in translation in Skype? That feature doesn’t do IM text, but does speech instead, doing both text transcription and translation at the same time. I’ve only seen it demoed and tried some basic French, and it seemed to work quite well for that. I am curious how it works in real-world conditions though.