My colleague's article was translated by another person and republished in the translated language. My colleague became the first author, and the translator became second author. Did my colleague or the translator thereby plagiarize the original article, as they published the same paper twice?
Publishing a translation of your own article is not plagiarism, or any other kind of misconduct, as long as it is clearly identified as a translation. Usually this means that the first page of the article says something like "Translation of [original title], published in [journal, volume, date]". The editors of the new journal should be aware that it is a translation, not a new paper. The publishers of the first journal should also give permission (this may be legally required if the author has transferred copyright to them).
The author should not try to "count" the translation as a separate paper, or otherwise mislead people into thinking that they are two independent papers. For instance, his CV should show one paper as a translation of the other, or list them together as a single publication.
However, translators are not normally credited as authors, unless they have made an intellectual contribution to the text of the paper, beyond simply trying to translate it faithfully and idiomatically. Likewise, the translator should not try to take credit for the translated paper as a publication on a CV, etc (unless as a separate section from papers they authored).