This module is for templates in multiple human languages, so the user can get a different output from a template depending on which language they select.
I spotted this module a day or two ago after having spent a little time doing exactly this job, making some HTML pages for different human languages. I had been using the plain template toolkit and found it worked pretty well.
I looked at this module wondering what advantages it offered. However, having read the documentation and source code, and written a test script, I don't think there is very much this module does which the plain template toolkit itself can't. Using the example from the documentation, the template toolkit can do much the same thing, if we just throw a variable "lang" into it:
[%- IF lang == "en" -%]
[%- ELSIF lang == "fr" -%]
[%- END %]
The advantage of the template toolkit is that it has the [% %] syntax, which I much prefer to the HTML-tag-like format of Template::Multilingual. Things which look just like HTML tags could be confusing stuck in the middle of real HTML. Template toolkit syntax stands out from HTML, and you can look at an unprocessed file in a browser, without the browser deleting the tags. Template::Multilingual's pseudo-HTML tags will just disappear.
The only thing which Template::Multilingual seems to offer above the template toolkit is the ability to parse strings like "en_US" and find default values like "en" when US English isn't available. That isn't particularly useful for me, but it may be for some.