When America becomes American, and Earth becomes Earthling, a suffix has been used. Is this slang or are there standard rules for this type of suffix? Most often I see/hear -ite, -an, -van or -er. I'm not a linguist. I write sci-fy and care about word abuse. Please be gentle.
There are not standard rules in the sense that you can predict what suffix will be added, but when a suffix has been added, it will almost always be standard, in the sense that there are other words using this same suffix. We use models in creating new derived forms. There seems to be some tendency to match up the source of the word and the suffix: earthling and venusian/terran, but not earthian or venusling.