Answer
Two things need to be in place. First, in the 'Used Language' window, tick 'Enabled for Login' for the language you want (e.g. en-GB) — this is what makes that language actually appear as a selectable option in the login language dropdown in the first place; without it, the dropdown won't offer it regardless of the reconfiguration setting. Second, to manually force the language-selection prompt to appear at any time (rather than relying on the reconfiguration default), hold Shift while starting the client — this brings up an explicit language-selection screen. The application will only actually launch in the selected language if the corresponding language resource files for that language are present in the client/runtime folder, so confirm those files exist for en-GB as well.