I have a domain .com but the site is in portuguese. How this can affect my reputation with search engines?
Dot com sites are not necessarily English anymore. In fact, there are Chinese language sites registered as dot com's and English language sites registered as .cn. While it is unusual, I cannot see a reason why it should effect your performance and here is why.
Regarding search, the results are not selected by domain TLD's but by semantic search using the requestors language. If your site is in Portuguese, it should perform the same as any other site of the same language. At one point, there was an assumption that the ccTLD meant something regarding language, but that seems to be fading fast. In fact, Chinese companies are finding value in bypassing the ccTLD .cn and registering .com sites and hosting them within U.S. or European IP address spaces. This is because of the poor reputation that the .cn ccTLD and Chinese IP address space has.