"Smart" login redirection?

Hmm...that's definitely an idea. However, currently there is a single menu item that is just an icon of a human bust silhouette that opens a login modal. The site landing page doesn't have a Spanish version, though GTranslate is active on the entire site, so they could use that to get started. But I was thinking just take them to the Spanish part of the site after logging-in. That's actually where it starts to matter. But this does give me another option, so thanks!

That's not typically how you'd multi-lingual Joomla. Normally you'd have the language switcher plugin enabled and the language module setup so they can toggle language. Next you'd have menu items for each language and modules (if they're language specific) for each language. You shouldn't need anything beyond that then as you'd have proper multi-lingula menu and module setup going on.

I have both the "Language Filter" plugin configured and enabled to auto-switch based on browser language and the "Language Switcher" module published and positioned to the right in my Gantry (template) Navigation section. I don't see any flags unless I also publish the GTranslate module in that same position. I fear that GTranslate will conflict with those Joomla-native extensions, since GTranslate is also setup to use the browser's preferred language.

We do have plans to duplicate the English areas of the site in Spanish, but what I was looking for was a way to use a single login dialog to redirect a "Registered" user to the language-correct landing page after login. It needs to be after login because, until then, we don't know if this is a "Student" (the only use for the "Registered" access level) or one of the site's authors or administrators.

You shouldn't need a Google translate module unless you don't want to use Joomla's multi-lingual functionality. Browsers these days also generally come with translation support built in. So that may not be necessary at all anymore.

We do have plans to duplicate the English areas of the site in Spanish, but what I was looking for was a way to use a single login dialog to redirect a "Registered" user to the language-correct landing page after login. It needs to be after login because, until then, we don't know if this is a "Student" (the only use for the "Registered" access level) or one of the site's authors or administrators.

Only way to do that outside of setting up the full Joomla multi-lingual menu and module systems is CB Auto Actions. Not aware of a way for Joomla itself to handle that otherwise.