Will an alien's debt in foreign country be a problem for his/her U.S. immigrant or non-immigrant visa application?

An unpaid car loan should not prevent you from being approved for a non-immigrant or immigrant visa. The Consulate is looking for your lack of immigrant intent and your assets, ties, and residency still in Brazil for the non-immigrant visa. For an immigrant visa, the Consulate is looking that you qualify, and are admissible, no bars, no criminal record, etc., and that you won't be a public charge in the U.S. Typically, there, you would have an employer or family sponsor who would fill out forms that prove financial support in the U.S. that they would back you for so that you do not become a public charge in the U.S. An unpaid car loan, unless it rises to the level of a crime in Brazil that would somehow make you inadmissible, shouldn't be significant enough for a denial-- and there are WAIVERS, so you might even for a non-immigrant visa, and if that was a minor crime, have a waiver of it. Again, if you had no assets so that you appeared to be a "risk" on immigrant intent, then, it could lead you to a denial for that reason-- it would depend on the "big picture" of your total assets, income, family, ties to Brazil.

The quick answer is "most likely not". The question is whether this unpaid debt could somehow render you inadmissable because somehow it could result in a finding of becoming a "public charge". USCIS has a fact sheet that discusses this analysis more in depth at http://www.uscis.gov/files/article/public_cqa.pdf . This analysis will be conducted by the US Consul in your home country. Typical evidence to help show that you will not become a "public charge" would be bank statements evidencing your ability to support yourself during your visit or stay in the US for other activities i.e. studies and other evidence of financial wherewithal i.e. deeds to property, ownership of business, stable employment etc.

If you apply for a green card the same analysis will be made. We learned at the June 2009 AILA Annual Conference in Las Vegas that DHS has a contract with Choice Point for public record information which may include information on your credit reports. In family cases, your family member is committing to be financially responsible for you using the I-864 Affidavit of Support. In employment based cases, your employment is evidence of your ability to support yourself at or above the applicable poverty guidelines.

I have never seen a case denied for finding of public charge resulting from an upaid debt here in the US or abroad.

I would add that the embassy screening on financials would not be limited to "public charge" issues. On a non-immigrant visa, depending on the category, the analysis would also address ties to the home country, sufficiently established, lack of need for a student loan or to work in the U.S. if it is a student category visa, etc. Rarely is an answer "simple" or "quick" in immigration law.