A few review feedback items (Frank, Hugo)
2. Move the following
"There cannot be more than one extension with the same singleton (u, t, ...). The private use extension must come after all other extensions."
up to the paragraph right under http://unicode.org/repos/cldr/trunk/specs/ldml/tr35.html#Unicode_locale_identifier. Prefex "As often the case, the complete syntax can't be completely captured by ABNF."
3. For equivalence, I think it is not a good idea to maximize "und", or perhaps even "und-CH", etc. The data for tags starting with und is mostly for other purposes (fallback).