For this type of styles we sometimes include the title in both languages, so that they're easier to find by search, see e.g.:

https://github.com/citation-style-language/styles/blob/master/universita-di-bologna-lettere.csl
<title>University of Bologna - Liberal Arts College (Università di Bologna - Facoltà di Lettere e Filosofia) (Italian)</title>

https://github.com/citation-style-language/styles/blob/master/european-journal-of-ultrasound.csl
<title>European Journal of Ultrasound (Ultraschall in der Medizin)</title>

https://github.com/citation-style-language/styles/blob/master/veterinary-medicine-austria.csl
<title>Veterinary Medicine Austria (Wiener Tierärztliche Monatsschrift)</title>

https://github.com/citation-style-language/styles/blob/master/the-institute-of-electronics-information-and-communication-engineers.csl
<title>The Institute of Electronics, Information and Communication Engineers (電子情報通信学会)</title>

Alternative, we could create a dependent for the other locale, and fixed both versions to their respective locales.

I guess the German title would be "Universität Heidelberg - Medizinische Fakultät Heidelberg" (there is a "Medizinische Fakultät Mannheim", and I'm not sure it uses the same style; https://www.umm.uni-heidelberg.de/home/)

The VetMed Austria style is the one that got me into CSL in 2011 and then reintroduced me couple of months ago to fix some outstanding mistakes. ;)

I also have been adding an XML comment to polyglot styles, so it's clear later why certain styles don't have a "default-locale". See https://github.com/citation-style-language/styles/search?utf8=%E2%9C%93&q=polyglot (an example is <!-- Polyglot; used both with German and English -->)

@damnation333, if you leave out the "default-locale", I would change this to <locale xml:lang="en">, otherwise people who use it with "en-US" won't see the effects of this locale block.

Less important, but you can just use xml:lang="de" here, too.