There are two parts to Dimensions, the indirect implementation and the direct implementation. The indirect is the Java source code for Dimensions, implementing the API. The direct is a group of patches for the vanilla Minecraft server. Using these, we can comepltely open source Dimensions without open sourcing Minecraft.
Can I write plugins in other languages?
This will be one of the major differences between Bukkit, Spigot, Sponge, & Trident. Dimensions will have official support for a variety of languages, but the main one will be Java.
Will this be the fastest server ever?
Probably not, I'm not worried about performance that much right now. This may change later down the line, when more is done. Dimensions is built on top of the Minecraft server, so it'll probably be faster than Sponge.
Work has been flowing out rapidly the past few weeks and we're almost ready for our first working build! All the source code is on our GitHub page, so be sure to check it out!