OK, here's a tentative initial framework. Comments please!
- Proto-Galach (Now to ??)
English-Russian mixed language/creole phases (extending through period of colonization of the solar system.)
(What's a good estimate for this period, 500 to 1,000 years? When will the first colony ship go out?) - Ancient Galach (11000 B.G. - 2000 B.G. = First interstellar colonization through foundation of Landsraad?)
(This covers a period of 9,000 years, so further divisions will be necessary!)
Language(s) of the "Slow Diaspora Period", characterized by dialectal divergence due to increasing physical isolation of speech communities and slower communications. (Virtually no travel and little communication between distant systems.) - Old Galach? (?2000 B.G. - 0 A.G.) The establishment of the Landsraad implies some faster form of transportation and/or communication and increased contact between formerly isolated peoples/languages. Over time this reduces dialectal differences. (During this period the name "Galach" first comes to be used?)
- Middle (Imperial) Galach (0 A.G. - 10217 A.G.)
(The OCB is compiled sometime around the beginning of this period, and while not universally accepted, it provides a ready exemplar of a new standard for the language. (= Classical Galach?))
Again, this will need to be divided further. Guild-fostered improvments in communication and transportation and a centralized Imperial government will further erode (but not completely destroy) dialectal differences and help create a conservative standard variety which will continue to change, but slowly.
- Early Middle Galach (0 A.G. - ?? A.G.)
- Late Middle Galach (?? A.G. - 10193 A.G.)
- Atreidean Galach (10193 A.G. - 13725 A.G.)
[Edit: Let's go with Atreidean instead of Lanthanian. ]
- Early Atreidean Galach (10193 A.G. - ?? A.G.)
The turmoil at the beginning of this period (Muad'Dib's Jihad) will no doubt see a massive influx of Fremen words into the language. - Late Atreidean Galach (?? A.G. - 13725 A.G.)
Leto's clamping down on population movement would probably foster increased dialectal divergence across the Empire, with the variety spoken on Arrakis being the "standard". (Leto himself, by his presence not necessarily by fiat, might have exercised a restrictive effect on the Arrakeen dialect?)
- Early Atreidean Galach (10193 A.G. - ?? A.G.)
- Post-Atreidean Galach (13725 A.G. - 15240 A.G. & onwards)
The population movements during this period will no doubt have an immense impact on the language. New dialects develop in the Scattering. (NB. The following are branches, not consecutive periods.)
- Old Empire Galach
Dialects spoken in the Old Empire - Scattering Galach
Dialects which develop out in The Scattering
- Old Empire Galach
[2/17/08 edit: finally figured out how to do lists!]
[2/19/08 edit: incorporated some DE designations]
[3/30/08 edit: minor edits to dates at beginning]