Testing correct mirroring of a GUI. Two GUI specifications are received, one that specifies text elements in a left-to-right natural language and another that specifies text elements in a right-to-left natural language and corresponds to a horizontally mirrored counterpart of the first GUI specification. For each child element in the first specification a start position, width, and a width of its parent GUI element are determined; for the corresponding specification a start position and width for the counterpart element is determined, a horizontally mirrored start position is calculated for the child element, it is determined whether the start position and width of the counterpart child GUI element are within a predefined tolerance of the calculated horizontally mirrored start position and the width, respectively, of the child GUI element; and, if not, the second GUI specification is updated with the calculated horizontally mirrored start position or width.