Hi Jens,
I don't think it has anything to do with the template engine, unless you expect F3 to render the xsl stylecheet as well. Otherwise, the xslt is just a file served by your web server, like any static file and not processed by F3.
In that case, the only issue I can imagine would occur if you were running the application in a subfolder (www.mysite.tld/subfolder) and forgot to prefix the xsl location with @BASE:
<?xml-stylesheet type="text/xsl" href="{{@BASE}}/xliff/ui/en-ru.xsl"?>