The multilingual layout is part of the normal Colemak experience, yes – on AltGr which again has dead keys (which are like in the olden days but with accent deadkey -> letter instead of the backspace). It's good enough if you need to write 'coöperate' or 'hæmatoporphyria' or 'passée' or 'Øystein' every now and then, and covers an impressive number of scripts even with its many unmapped positions (that Shai put tildes on).

I didn't like it much though, so I made the Colemak[eD] mappings where the dead keys are all on AltGr+symbol keys and the arrangements are different otherwise. Like Shai's setup, my mappings cover lots of scripts but I feel my layout is more intuitive (to me – big surprise!?). ;) Unlike Shai's setup, I didn't try to cram in a great deal of accented letters on AltGr mappings as we have the dead keys for those anyway.

However, as davkol says that's not quite enough for the locals! In my experience, if you use a letter or symbol around 1% of the time or more it gets annoying to use AltGr for it. So I keep brackets and the less/greater key for locale symbols ('æøå'/'äöå' for Scandinavia, 'äöü' for German etc etc). They are too frequent in our own languages to be using dead keys for the needed accents. For some locales where for instance the acute and grave accents are used much and for many letters, I instead added a dead key without AltGr mapping so it's still very easy to get the accented letters.

It's all in my Locale Colemak topic, if you haven't read that already.