I found the answer:
Go to Color at the bottom.
Select the clip you want to modify (the small video on top).
Add your usual nodes as needed.
Right click and Add Alpha Output. This will create a new blue terminator.
Right click and add a new Corrector node and use the rectangle, ellipse, or whatever shape you need to define the transparent/opaque regions.
Resolve and Premiere both support .srt files for subtitles, which is what you want instead of "Excel files or whatever." There are many online services that will generate .srt files from spoken audio, and timestamp them appropriately so that they sync with your video.
Templater Rig can help with this. It's a plugin that connects the text in your After Effects project to a Google Sheet or other spreadsheet. Bear in mind that each language will have different read rates and sentence lengths for the same meaning on-screen, so swapping the translations is only the first step to making the videos look ok.
Resolve supports scripting with Python, though you’ll need to write your own script. Fusion, included with Resolve, supports scripting with both Python and Lua. There aren’t really templates, per se, but you could probably hack something together by saving a fusion composition as your template and driving the file selection with scripts. I don’t think the ...