right, the contents of the %APPDATA/Spark/spark.properties file is basically spark saving it's settings from memory... there is no guaruntee of order in that file (due to how the Java Properties API works). That file may be over-written at any time during spark's runtime (every time a setting changes, or spark logs out, shuts down, etc).
The language attribute does not appear to be set by default... which implies english for spark. if you set another language, then the language= property appears in the %APPDATA%/Spark/spark.properties file.
So, you could take your "neutral" spark.properties file, and add:
language=<your language of choice>
anywhere in that file, even at the bottom is ok. Then, next time spark launches, it will read from that file and set the language to the one specified. that is... until a user changes it of course 
To figure out what the correct language property is, it may take some trial-n-error. Just launch Spark, set your language, then logout/shutdown spark. Check the spark.properties file and see what is set.
Examples:
English:
language=en
Deutsch:
language=de
Espanol:
language=es