Well, as a professional translator: agreeing to take down a translation if someone complains doesn’t make it any more legal or ethical to post it in the first place. Many writers are very concerned – especially if they do not have a reputation in the language that one is translating into. A “debut” of any sort with a poor translation can harm their reputation and does leave you open to legal action. “Only a blog” won’t hold water because, if anything, it is even more available to a google search.
You are only safe from this kind of thing if you are translating from countries that have not signed the international agreements (Afghanistan and some others I can’t remember off-hand.). While they are not laws, they are recognized agreements and will be enforced in the country you live in under its own copyright laws. A translation isn’t considered fair use. Although, having said that, I agree with everything you said about translation being a fantastic apprenticeship. – Sorry to be a wet blanket.