There are hardcoded style in include/layersmenu.js file:
function setleft(layer,x) {
if (DOM) {
document.getElementById(layer).style.left = x + 'px';
} else if (NS4) {
document.layers[layer].left = x;
} else {
document.all[layer].style.pixelLeft = x;
}
}
For RTL interfaces, we will need to change each "left" with "right", so it is better if this file move to modules/system/class/gui/legacy folder then can add it in translation patch or improve this function to be more smarty and useful for RTL interfaces.