During the Migratory period it seems like all of the tribes that settled in previously Roman territory completely adopted the language of the places they settled. Visigoths, Ostrogoths, Lombards and Franks all adopted the latin language within only a couple of generations. I understand this is probably a big question, but why did this happen with these particular tribes as opposed to say the Hungarians or Saxons who obliterated the local dialects during their own migrations?
Although my familiarity with the Romans and the Germanic tribes and areas is very limited, I would agree with your statement that the Germanic tribes didn’t adopt Latin in the places that they were majority.
That is a pretty hard question to answer. There are so many factors. So many different tribes/cultures have different values and cultures.
Many Germanic tribes had a lot of familiarity with Romans and Roman culture. Not only was war prevalent between the two, but trade was prevalent, diplomacy, etc. In fact, the Romans employed a lot of Germanic mercenaries into their military.
It is not uncommon for invaders to assimilate into local cultures, customs and languages. The reasons why, of course, vary. I would argue that adopting language and cultures of lands that were conquered are mostly for political and diplomatic reasons.
I'll give an example of the Normans when they settled in Normandy. Rollo negotiated with the king of France in regard to his settlement. Part of this deal included that the new Viking settlers had to swear to convert to Christianity. Overtime, the Norman settlement expanded. Overtime, they intermarried with the native French, they adopted French culture as well as the French language. By doing this, they realized that, it would make ruling over the native French easier and help establish their power. Because the Vikings were so used to plundering and pillaging - and not running a successful overseas state, they relied on surrounding French areas to help. Which is why they were so willing to adopt - purely to help them establish their power.
As you know, England was part of the Roman Empire. However, the Latin language was pretty reserved for nobles, military, and administrative purposes in Britannia. The Romans had been in other places in continental Europe far longer than they were in Britain, therefor their footholds were much stronger there. Once the Romans left, the Britons didn't have the Romans there to protect them from attacks from beyond. Once the Romans left, they native Britons were exposed, and extremely vulnerable. They no longer had the Romans for protection, what also didn't help the Britons was the fact that the Saxons were extremely bellicose. Because they were exposed and vulnerable, the Saxons were able to impose their influence much stronger, and blood shed was significant. Also, if the Britons weren't being killed off, they were being displaced. The displaced native Britons fled across the channel and ended up in a place that is current day Brittany. And yes, that name Brittany is a result of the fact that many Britons settled there.
So, because the Britons were exposed and vulnerable after the Romans left, and because so many Britons were displaced as a result of the Saxons, the Saxons were able to influence the area VERY heavily.