After seeing this remark in functions.php from Twenty Thirteen I think my first thought in this closed thread was right…
/* Translators: If there are characters in your language that are not
* supported by Source Sans Pro, translate this to ‘off’. Do not translate
* into your own language.
$source_sans_pro = _x( ‘on’, ‘Source Sans Pro font: on or off’, ‘twentythirteen’ );
The blog I need help with is netztaucher.wordpress.com.