Tag Localization: how to create a tag without translation fallback version?
OK, so this isn't quite how i expected it to work, but it does so far. I won't post code, because you just need the right process:

Assuming you're creating tags for a dual locale site of English (en) and Spanish (es)


Create a TagModel for first locale (en)
Set attributes and content for first en version
Save it
Load a new instance of a TagModel by loading the tag you just saved by id, but specify the other locale. 

$tag_es = craft()-&gt;tags-&gt;getTagById($tag_en-&gt;id, "es")

Assign the es title, content and attributes (regenerate the slug, too!)
Save the es tag with 

tags-&gt;saveTag($tag_es)



Now, Tag Manager should keep them separated by locale, and you can query them by locale, too.

The bottom line is, you have to create alternate locale versions by essentially cloning the original Tag Model while specifying the new locale string, then reassign the content and just save it. Again, not what I expected.

EDIT: Check your results in the database. Don't believe the Tag Manager — it only shows data from one locale for some reason.