Yes, Singular overlays can be displayed in a variety of different languages.
When building or adjusting an overlay theme in Composer, you can choose a specific font for your text elements. Within the Text Widget in the Property Panel, you can scroll through font options and search for the font that supports your language. If you hit "More fonts ..." at the bottom, you can search for additional fonts in the Font Browser.
Font Options in Text Widget
If you can't find a font you are looking for, we recommend downloading a font from Google and then submitting a request with the font file attached so we can add that font to your account.
On the Google Fonts website, you can search for a free font by language. Check out the website, here.
Google Fonts, searching by Language
Google has also been developing a font family called Noto, which "aims to support all languages with a harmonious look and feel". Check out this font family from Google, here.