I'm definitely slower at learning the language than I was the last time that I moved abroad, 25 years ago. On the other hand, Norwegians are generally pretty fluent in English, and our office defaults to English, as we are perhaps 10-15% non-Norwegian. But on the other hand, I probably would be learning faster if that were not the case...
Regarding employment within the US: if you have an H-1B visa, bear in mind that it's fairly straightforward to change to a different job with one. I've done a transfer for a new hire at a startup once, and know others who have done so a number of times.