Files containing the first <switch>-Element after https://www.mediawiki.org/wiki/Manual:$wgSVGMetadataCutoff (> 256 kb), are not recognized as translatable-SVGs, to avoid excessive resources by xml-parser.
So adding
<switch> <g systemLanguage="de"></g> <g systemLanguage="en"></g> <g></g> </switch>
at the beginning (containing all used langues) solves the issue