Why not use "ss" instead of "ß"?
It's - in my opinion - an useless character anyway.
To give you further advice:

As Sphax' Font is made out of capital letters you should use "ss" instead of "ß" anytime. It's the same in German. You don't say "FUßBALL" but "FUSSBALL".

Just my two cents.

1. The Sphax BD font is NOT only capital letters!
2. This affects many MC translations - it's not just an issue for German.

The problem is caused because the texture pack doesn't include the new font files (glyph_00.png, glyph_01.png etc). Including them all (all Unicode chars) would be a huge job, but just adding glyph_00.png would add ISO 8859-1 (Western Europe) support.

Sphax BD (default.png) currently supports only these characters:

Ç ü é â ä à å ç ê ë è ï î ì Ä Å
É æ Æ ô ö ò û ù ÿ Ö Ü ø £ × ƒ Ø
á í ó ú ñ Ñ ª º ¿ ® ¬ ½ ¼ ¡ « »

(i.e. old IBM-PC/MS-DOS code page 437)

glyph_00.png contains these characters:

¡ ¢ £ ¤ ¥ ¦ § ¨ © ª « ¬ ® ¯ ° ± ² ³
´ µ ¶ · ¸ ¹ º » ¼ ½ ¾ ¿
À Á Â Ã Ä Å Æ Ç È É Ê Ë Ì Í Î Ï
Ð Ñ Ò Ó Ô Õ Ö × Ø Ù Ú Û Ü Ý Þ ß
à á â ã ä å æ ç è é ê ë ì í î ï
ð ñ ò ó ô õ ö ÷ ø ù ú û ü ý þ ÿ

(i.e. ISO 8859-1)

CP 437 dates from the early days of personal computers and doesn't even support Western Europe well.

How Minecraft chooses which font scheme (default.png with substitutions or only glyph_nn.png) to use is a little mysterious. It seems to be something to do with the frequency of characters not found in default.png as Spanish (as well as Italian and German) use the Sphax font, but French and Irish do not, even though Irish only uses a subset of the characters used in Spanish.

It would be great if Sphax BD could be expanded to include languages covered by ISO 8859-1.