Are all Nexus 5 phones sold unlocked? 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.