It's not a language at all.
It's what happens when one switches to graphics mode but doesn't wipe the text mode character+attribute cells from video RAM. The display varies according to exactly what graphics mode one has programmed the display adapter to use. There is a range of possibilities, depending on such things as banking, RAMDAC width, palette registers, and so forth. In your particular case it further depends on how closely your virtual machine matches the behaviour of real hardware.
Depending on where in the bootstrap process this is, what operating system it is, why the display adapter is in graphics mode, and what the operating system proceeds to do, it may well be that your software/firmware still thinks that the display adapter is in text mode, and is busy writing text cells to the tiny portion of the framebuffer that is used by text mode. In such circumstances one sometimes sees the pixels flicker. (One of my more … erm … interesting systems programming debugging experiences was running the
dir command in such a state and discovering that the system was still alive from the way that the pixels flickered.)
The "weird background pattern" is simply what was left in the framebuffer from the last time that the display was used in graphics mode. It looks odd merely because the last graphics mode wasn't the same as the graphics mode that your display is now in.