To add localization support for your plugins, you can probably follow the example of the DShowPlugin, generating a LocaleStringLookup from localization files in a locale subdirectory of your plugin directory (see the LoadPlugin() method of DShowPlugin.cpp).
However, as for adding plugins to CrowdIn...that's another matter entirely. The CrowdIn data is what will be committed to git, so having non-core plugins intermixed with core translations could get hairy.
My recommendation would be for plugin owners to a) make sure their code is open source, preferably on GitHub, and then b) make their own CrowdIn project, and apply for an Open Source license so that you can use it for free. That way, plugin owners can have control over their localization, can upload strings at will, and do their own quality control. I'm sure we could find a way to promote translation of plugins alongside translation of OBS itself somewhere on the site, if plugin owners apply for it.
Those are my thoughts off the top of my head. We can probably discuss it more. I definitely want to see more plugins get translations, and I'm sure there are many others who are eager to help, but I do think it's important to keep things organized where they should be.