auto-commit: translation updates
fix attempted double-translation ;) (thanks @shadmar)
run optipng on all PNG files to optimise and fix libPNG warnings

Recent versions of libpng (since 1.6 I think?) print a warning to
stderr when loading most of our PNGs. The warning is:

libpng warning: iCCP: known incorrect sRGB profile

and for some PNGs:

libpng warning: Interlace handling should be turned on when using
png_read_image.

This commit runs optipng on all our PNG files with this command:

  optipng -o2 -nc -nb -i 0 -strip all $IMAGE

Which strips the profile data from the image and rewrites it.
This fixes all the warnings, at the cost of generating a huge
commit and touching all our files.
replace remaining references to English
return nil to Lua if the requested resrouce wasn't found
use global gray colour for speedlines

the mxe build was consistently initialising LINE_COLOR to 0,0,0,0. it
appears to be some sort of initialisation order thing but I can't
figure out why its doing it. this will do
auto-commit: translation updates
Merge branch 'master' of https://github.com/pioneerspacesim/pioneer into per_frame_profiling