As a spanish developer, internationalization was one of the first things I cared when I started coding plugins and themes for WordPress and I still do. However, it doesn't end there: RTL is another important feature for hebrew, arabic or persian languages and is many times overlooked or implemented scarcily, sometimes only in admin interface, others only in front end.

Lately there's another issue with the introduction of Google Fonts: designers tend to use Google Fonts that don't have the proper subset for special characters found in some languages. First one that comes to my mind is Czech Republic and its language that requires the latin-ext subset. Many themes include only the latin subset leaving some languages to render special characters using system fonts. Result: uglyness. What's worse, sometimes themes include a font that doesn't have the subsets available at all with no fallback to a font that does have them. For example, the Oswald font has latin and latin-ext subsets, but good luck typing something in cyrillic or greek. Theme vendors adding Google Fonts should empower users to add/change fonts with the proper subset, be it a simple filter or a complete user interface, something that allows them to correctly render their language.