Because the "lang" attribute isn't always informed, I simplifie Perfect display target link language (hreflang) to a rules who doesn't take care about the text language, except where the language is specified and is the same.
Advantage: It works on most sites.
Disadvantage: when the attributes are well informed, it displays all (unless the language is specified), it can be tiresome. And and this rule does not cover all scenario.

Because the language can be specified with an attribute Regional (e.g. "en-us"), I use CSS3 selector who allow to specify "who start by" : ^. So it can make problems on browser who don't support CSS3 (but yes, firefox 3.* support CSS3)
If these rules pose problems on some sites, say it here and I add exclusions