Correction, it starts, then fails (icon removed from dock!), then X11 starts with the message.
Also, here's some debug output:

dr@elin:/Volumes/XCSoar $ XCSoar.app/MacOS/xcsoar
Free storage 0 KB
Starting XCSoar 6.2.3-UNIX
Initialise application instance
Create main window
Initialise graphics
Initialise fonts
Font initialisation failed
Warning: locale not supported by Xlib, locale set to C

Open a terminal, and type "ls /Library/Fonts/?", paste the output here.

Replying to the e-mail apparently didn't work (or I'm not patient enough), so:

Sure, and a few more places with fonts follow.

/Library/Fonts?:

Apple LiGothic? Medium.ttf
Apple LiSung? Light.ttf
Courier New
Courier New Bold Italic.ttf
Courier New Bold.ttf
Courier New Italic.ttf
Courier New.ttf
Devanagari Sangam MN.ttc
Georgia Bold Italic.ttf
Georgia Bold.ttf
Georgia Italic.ttf
Georgia.ttf
Latin Modern
Microsoft
Myanmar MN.ttc
Myanmar Sangam MN.ttf
Times New Roman Bold Italic.ttf
Times New Roman Bold.ttf
Times New Roman Italic.ttf
Times New Roman.ttf
Verdana Bold Italic.ttf
Verdana Bold.ttf
Verdana Italic.ttf
Verdana.ttf
encodings.dir
fonts.dir
fonts.list
fonts.scale

===

~/Library/Fonts?:

Futura.dfont
GillSans?.dfont
Optima.dfont
Palatino
encodings.dir
fonts.dir
fonts.list
fonts.scale

===

/System/Library/Fonts?:

Apple Braille Outline 6 Dot.ttf
Apple Braille Outline 8 Dot.ttf
Apple Braille Pinpoint 6 Dot.ttf
Apple Braille Pinpoint 8 Dot.ttf
Apple Braille.ttf
Apple Color Emoji.ttf
Apple Symbols.ttf
AppleGothic?.ttf
AquaKana?.ttc
Courier.dfont
Geeza Pro Bold.ttf
Geeza Pro.ttf
Geneva.dfont
HelveLTMM
Helvetica LT MM
Helvetica.dfont
HelveticaNeue?.dfont
HelveticaNeueDeskUI.ttc
Keyboard.ttf
LastResort?.ttf
LucidaGrande?.ttc
MarkerFelt?.ttc
Menlo.ttc
Monaco.dfont
STHeiti Light.ttc
STHeiti Medium.ttc
Symbol.ttf
Thonburi.ttf
ThonburiBold?.ttf
Times LT MM
Times.dfont
TimesLTMM
ZapfDingbats?.ttf
encodings.dir
fonts.dir
fonts.list
fonts.scale
ヒラギノ明朝? ProN W3.otf
ヒラギノ明朝? ProN W6.otf
ヒラギノ角?ゴ ProN W3.otf
ヒラギノ角?ゴ ProN W6.otf

Please try this new build: ​http://max.kellermann.name/download/xcsoar/devel/osx/XCSoar2.dmg

It runs, though the choice of font is really odd (see screenshot). Antialiasing doesn't work (should it?), and the font is not very readable. However, this runs sufficiently well for debugging and testing.

I assembled a rather random list of fonts that are supposed to be present on Mac OS X, but the best priority for choosing one is subject to further optimisation. Also, anti-aliasing is currently disabled, and needs some serious testing and hacking before we can enable it. Fonts have been a very low priority for us; if you care, write a feature request ticket, asking to enable anti-aliasing.