The localized label for calculator takes about as much space as the field meant for the user's expression -- doubling the size of the applet on the panel. Maybe, it should instead be shown inside the field -- until the user types something? I think, the modern term for this is "placeholder" or "watermark". Although there is a debate among user-interface designers on their general use: https://ux.stackexchange.com/questions/80911/is-it-better-to-label-input-fields-watermark-them-or-both in this case it seems a clear winner...
The clear winner seems to be doing both..? I think that's not easily feasible for a panel plugin because it would have to change its size request when suddenly showing the label instead of the placeholder, moving all other items in that panel around.