These days, do korean/japanese people actually get offended when you speak informally?

You always hear about how in japanese and korean,you should speak formally for elders and strangers. But for a learner of korean/japanese, would the same respect be needed? would an elder feel upset if you spoke as you would with you friends ( not slang of course) . When I went to chile, my teacher told me that the chileans will understand that wwe are just learners and its easier to use informal language, so they wouldent get offended