If you want to publish the translated version of the article, or share it publicly, then yes, you absolutely must ask permission; to reuse others' academic work publicly without permission would be academic misconduct.
If you are just translating it for a colleague's personal use then it's fine (for example, once I saw a Japanese article which looked relevant to my research, based on the English abstract, so I asked a colleague to translate the methods and results for me just so I could know what the study did).