1. A computer system for testing correct mirroring of a graphical user interface (GUI), the computer system comprising: one or more computer hardware processors, one or more computer-readable storage media, and program instructions stored on one or more of the computer-readable storage media, which when executed cause at least one of the one or more computer hardware processors to perform a method comprising: receiving a first GUI specification in a user interface markup language, which includes at least one child GUI element and its parent GUI element, wherein the first GUI specification specifies text elements that are in a left-to-right natural language and which corresponds to a horizontally mirrored counterpart of the first GUI specification, and wherein the second GUI specification is provided in display-independent units; receiving a second GUI specification in a user interface markup language, which includes a counterpart for each GUI element in the first GUI specification, wherein the second GUI specification specifies text elements that are in a right-to-left natural language and which corresponds to a horizontally mirrored counterpart of the first GUI specification, and wherein the second GUI specification is provided in display-independent units; for each child GUI element in the first GUI specification: determining from the first GUI specification, a start position, a width, and a width of its parent GUI element; determining from the second GUI specification, a start position and a width for the counterpart of the child GUI element; calculating for the child GUI element, a horizontally mirrored start position, based on its start position, width, and the width of its parent GUI element; determining if 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 in response to determining that the start position or the width of the counterpart child GUI element is not within a predefined tolerance of the calculated horizontally mirrored start position or the width, respectively, of the child GUI element, updating the second GUI specification with the calculated horizontally mirrored start position or the width, respectively, of the child GUI element; rendering on a display device, a horizontally mirrored GUI, according to the second GUI specification; in response to determining that the calculated horizontally mirrored start position or the width of any child GUI element is not within the predefined tolerance of the start position or the width, respectively, of the counterpart child GUI element, updating the rendered horizontally mirrored GUI, according to the updated second GUI specification; and outputting a report that indicates, for each child GUI element in the first GUI specification, whether the start position and the width of the counterpart child GUI element are within the predefined tolerance of the calculated horizontally mirrored start position and the width, respectively.