Frankish vowel shifts
Yes I just noticed that. I fixed it now in the other entries. I think the change ai > ei is part of umlaut, but there is other evidence (like *warjan) that umlaut didn't occur yet. It was probably purely phonetic, so the diphthong was [æi] like you said and was "heard" by Romance speakers as [ai] rather than [ɛi]. I don't know about *au. It shifted to ou at some time, and to ō in Old Dutch after that, and Old French borrows it as o too. I suppose it's "neater" if the change ai > ei is paired with au > ou, so we can just say neither of them happened yet. We can be pretty sure that it wasn't ō yet, because the original PG *ō became uo first; in the same way, PG *ē2 was still ē, not ie.
I wonder what Proto-Germanic *eu became. In Old Dutch, Old High German and Old Saxon it is io, but I think the earliest texts show eo and Old English still has eu in some early texts. So Frankish probably had eo or eu too.
I think that w:Sievers' law disappeared in West Germanic after the w:West Germanic gemination had taken place. The law determined whether a word had -j- or -ij-, depending on the weight of the root syllable. But the gemination caused all light roots to become heavy anyway (except those with -rj-), so the alternation no longer had any phonological trigger and probably disappeared. I'm not sure whether the end result was -j- or -ij-, but there are some hints.
- -ij- was already the usual alternant for heavy stems before the gemination, and it was also the more common one, so it would make sense that it became generalised once all stems were heavy.
- The fact that ja-stems have the ending -ī and never -i probably speaks in favour of the "long" alternant -ij- becoming the general form. This apparently also happened in the imperative of class 1 weak verbs, which always have -ī and never -i (which would have disappeared).
- I think it's also more likely that the -ij- would disappear in Old High German and Old English if it was a syllable, and some early Old High German texts actually treat it as a vowel or a kind of diphthong (willeo, willio), so it was probably syllabic and therefore -ij- rather than -j-. This actually fits with how Old Norse treats it. Old Norse had no gemination (except for k and g) but it lost -ij- while keeping -j-; this is exactly what happens in Old High German and Old English, too, except that -j- only occurred after -r-.
If you look at *hraigrō, you can see the /ai/ was still in flux during Old Dutch and Old French.
I know that PGm. *eu survived in some Frankish names as /eo/ or /ēo/, which might have sounded like [ɛːo]. I'll try and dig up a noun that made it to Old French.
Dutch is right on the border area between ē and ei. Most Dutch words have ē, but it seems that certain words, in particular those with umlaut, have ei instead. The words with ei become more frequent as you head east and south, and the form reger is also attested in Middle Dutch. With *hraigrō there is another explanation though: the -g- that disappeared may have raised the vowel before it. I think there are other words that have that (including native Romance ones).
uo doesn't stand for [øː] though, it stands for [uo]. uo developed in Old Dutch and Old High German from Proto-Germanic stressed ō in all words (OHG has guot for *gōdaz), and in a few also from wō (kuo (“cow”) < *kwō-, Old Dutch huo < *hwō). There is also a parallel change from ē to ie. I think the earliest Old High German texts have ua or oa, but most have uo. I think they also have ea and ia early on, and ie later. ([1] has ua and ia) So *muotan certainly came from *mōtan, not *mōtjan. But I'm not sure whether the change ō > oa/ua and ē > ea/ia happened already in Frankish.
Do you know if there is any evidence in Old French regarding this change? We know that Germanic au appears as ō, but what does Germanic ō appear as?
Well, if the same sound ends up in modern French as two different sounds, I wonder why that is. So I'd like to be able to compare other words that were borrowed from Germanic ō, to see if there is any general rule or pattern to them. And maybe through that, we can figure out whether Frankish still had ō, or whether it had already become a diphthong.
Frankish/Old Dutch th becomes d in (Middle) Dutch, so that can't be right. And Germanic eu becomes ie, not ū or ui, so that can't be right either.
I can't actually find any sources about that. I've found vlieke in a Middle Dutch dictionary, but that says it was borrowed from French rather than the other way around. The combination of -iu- and -kk- also bothers me. Normally -kk- is formed from -k- followed by -j- through gemination, but that only applies to light syllables and the -iu- makes it heavy. So it seems that the Germanic form must have been *fliukkijōn. I have no idea how that could have become the Middle Dutch word, though; the -kk- should have been preserved and -iu- normally becomes -u- in Middle Dutch, only -ie- in a few dialects. So maybe it was really *fliukijōn or even *fleukōn.
In any case, I don't think the evidence for this reconstruction is strong enough to warrant an entry. I'd prefer it be deleted.
You're right, looks like Du. vliek is actually a reconstruction from OFr., but it does have obvious Frankish origins, with many cognates, so it shouldn't be outlandish to reconstruct. http://gtb.inl.nl/iWDB/search?actie=article&wdb=ONW&id=ID3683&lemmodern=vliek