There is nothing special in translatable pages, except changing them generates work for translators :)
Yes, I agree, it is best not to change pages already set up for translation. That is why I thought that it might be possible to change the existing link at 'fallback language', without changing the text. Would it be possible to change the link using a bot, so that there is no extra work for translators?
If neither of my suggestions are very satisfactory, it doesn't matter; I'm sure we'll find a good place for this page eventually.
I have tried the page in Firefox 13.0.1 and Firefox 14.0.1 for all 4 skins and in internet explorer also. The messages supportedlanguages-summary and supportedlanguages-colorlegend always appear, but the supportedlanguages-localsummary appears in the English interface only (tested for cy and sw).