How rude is it to speak to a Japanese stranger informally instead of using the polite form?

Question

Are there any native Japanese here that could tell me how rude it is to speak informally to Japanese people I have never met instead of using the polite form? I'm assuming it's more rude the older the stranger is than me?

This came up recently when I was at a conference and ran into a couple of Japanese guys about my age (maybe in their 30's) and they seemed to speak less English than I was able to speak Japanese, so we mostly spoke in Japanese. It's been a few years since I've studied Japanese and I've forgotten quite a bit, but I used to have Japanese friends in college and became more comfortable using the non-polite form so that's what I seem to remember the best. But I did not want to offend these guys so I found myself pausing and stuttering trying to quickly remember how to conjugate things politely and the conversation seemed pretty rough and even then I would pop out an informal verb here and there. If I had spoken informally I think it would have been much smoother and in my mind that would have been better for all of us.

So until I get better, wouldn't it be better to say something impolitely than to not say anything at all? Would strangers care that much or would they at least be appreciative that you are trying to speak even if it's impolite?

Examples:

「どこからきた？」instead of「どこから来ましたか」

「これ何？」instead of「これはなんですか」

Answer 1

Language is about communication, first and foremost, but whereas in many languages (such as English) the focus is on getting the information in itself across as smoothly as possible, other languages (such as Japanese, German or the romance languages) require the encoding of social relationships along with the information. Why this should be so isn't necessarily important; but it is so.

In general, failure to include this information hovers somewhere between "acceptable given that you are a foreigner" and "quite rude indeed"; depending on the level of intimacy in the conversation.

In Japanese, these social requirements are far more formal than in most languages, but because of the overall structure of the language, there exists an "out". (These two things are probably related.)

This is not a permanent solution; the permanent solution is to learn how to conjugate verbs. But as a stop-gap measure while you bone up on your formal forms, attaching です (or でございます) to the end of your sentences can help lubricate social interactions. While it's not by any stretch of the imagination the right way to form appropriately formal sentences, it shows that you are aware of the appropriate level of formality, even if you don't have the means to express it correctly.

Edit:

With regards to specific examples: I can't speak to 「どこから来たか？」, but in general, direct translations will lead you astray. What verbs mean in different tenses vary from language to language; and what sentences mean idiomatically vary from language to language. As an example, if someone seems like they have a headache, you might be tempted to ask if their head is okay; 「頭大丈夫か？」. Unfortunately, when translated directly, this doesn't come across so much as "I notice that you appear to be having a headache, is there anything I can do to help?" as it comes across as "Are you nuts?"

Answer 2

どこからきた？ does sound rude, but things are complicated because not so rarely people would talk to you in non-polite forms.

Among Japanese society, there are opportunities when you can escape from duty of using Standard Japanese. When you are a child, when you belong to internal societies which dialectal speech is applied to and when you speak to them, in other words, when it's not guaranteed that both speakers are used to Standard Japanese.

I'm afraid that not a few people will unconsciously consider foreign people to fall into the same category.