For menu items defined in the config.toml, we do matching on URLs for IsMenuCurrent etc., so for multilingual sites I guess you will have to define the full URL path, so:
[[languages.en.menu.main]]
name = "About"
weight = 2
identifier = "about"
url = "/en/about/"
And then just:
<option value="{{ .URL }}"
This will be simpler in the next Hugo version, where you can add front matter to the home and section pages etc.