<Nice explanation of the help browser works cut>
Okay, I have some idea how this all might work.
Regarding the "topic not found"-fallback: I think we should simply add some hidden texts in every language which can be mapped as fallback. Those are strictly required for every language and should be part of the criteria for inclusion in the official sources. We just need to negotiate the identifier for the link. If the some link cannot be mapped an automatic fallback to the fallback indentifier should happen. We can semi-automatically ensure that the help module covers all possible topics and print missing ones (in some languages) by some tool which is written in almost no time.
Regarding the languages: Yes, let's do it as so say with the names and canonicalize "C" to "en" or "en_EN" or "en_US" or whatever and map it in the help.
Regarding the fallback: Your suggested fallback implementation sounds good to me (stripping down the localename, that is). I, however, would add a preference option whether to fall back to English or rather display an error in mother tongue. As you said a fallback to English might be a bigger problem than no help at all in some countries and at least it's less confusing than a mix of several languages.
Description: This is a digitally signed message part