Maybe Moving Pictures can split the job between Imdb+ and the TMDb scrapper, using the synopsis from TMDb according to the user language and everything else handled by IMDb+. What do you think?
The TMDb scraper adds the IMDb tt-ID for the majority of movies (as long as the info is availabe). That is why IMDb+ can take over for those movies imported. It is already possible with IMDb+ to only refresh parts of existing movies (missing items, score/rating) and skip the fields that normally do not change (title, crew, summary). The rename system works seperate from this, so it will still apply when enabled.
That means you can import all your movies with TMDb (you have to like temporary disable IMDb+), and get all the Spanish info, and then let IMDb+ take scraper-script control of those movies (option via IMDb+ plugin) and make sure it is configured to not update all the fields.
It is an ugly workaround, and there is not an easy way to automate it, but you can at least do it for your entire collection at once.
Converting the entire rename database system into Spanish is also already possible with the custom rename database (just a lot of work).
Try it on a couple of movies otherwise, see if that works for you.