Are all Nexus 5 phones sold unlocked (i.e., not tied to a specific carrier)? If I buy a Nexus 5 from a store in Germany will I be able to use it in another country (Brazil)?
Yes, at this point. But, there's a talk about Sprint-locked Nexus 5.
Note: If you intend to use 4G LTE, then you need to check LTE market-compatibility, too. Other than that, you're good to use the phone in both countries.
Yes, currently. However this could change, the US carriers sometimes sell locked phones.
There is currently only one Nexus 5 variant, so you don't have to worry about LTE compatibility issues.