Is it legal to use translated comments from other sites?
Depends upon the terms and conditions set forth by the website whose content you are using. Most sites, by default, don't allow others to use their content without their permission. Other sites, like the Stack Exchange sites, are released under a creative commons license.
If the content you want to use is on sites which do not explicitly allow you to use their content, you cannot use it - but that doesn't mean you can't ask for permission.
Will using this content adversely affect my site's search engine rankings?
Google "penalizes" duplicate content by filtering out the duplicates and displaying only the original in its search results. It's not a penalty per sé: Google doesn't want the same content appearing multiple times in their search and they want to make sure the original publisher gets proper credit for their work. The worst thing that can happen would be for the pages that have duplicated content to fail to appear in search results.
In your case, I believe the fact that you are translating the content to another language will prevent the content from being flagged as duplicate content. Yes, the content is the same in meaning, but in order to reach non-English audiences (assuming the original content is in English) it is to be expected that the content will be translated.
If this were not the case, multilingual sites would fail to appear in search results because users would only be able to find content in one language.