Add (random) language showcase

**Short description**: A block that showcases one, two or three languages that need translations help. Randomly picked between the languages that have a progression between 50 and 99% (for the App resource).

**Location**: Homepage

**Why have this**: Showcase languages that need translations, but aren't in so high need that they're demoralizing, so that the visitor thinks "Ah, let's contribute".

**More info**: Clicking on the language should open the respective language in a new tab. A 'More info' link should open the new Translate contribute page (#102).