Different font sizes reset by slider bar

I have a multi langauge app, and the font we are using for the local language is significantly smaller than the trade language - this looks very odd in the (excellent!!) two-pane view, which we’d like to keep as the default.

I’ve edited the body style of the local language to by 30px (trade langauge unchanged at 20px), which looks fine when the app is first opened. However, as soon as you resize the text with the resizing bar, the font size difference again becomes apparant. It seems like the resizing bar makes both languages the same font size.

Ideally I would like the local language font to be 50% bigger than the trade language (whatever this value), throughout the range of the resizing bar. The font size doesn’t seem to accept a percentage value for the body style though. Is that possible?