There is some help from the fact that all the romance languages (languages derived from Latin) have similarities, so if you understand one of Portuguese, Spanish, French, Romanche (used locally in Switzerland and closest to Latin), Italian and Romanian, it isn't much of a step to learn the others. Similarly, English is derived from old German and 52% of English words are of German origin, so it should be simple if you know English or German to learn Dutch, Afrikaans, Norse, Icelandic (which is Norse from 1000 years ago), and a bit more of a stretch to Swedish, but they are all related. Finnish , Estonian and Hungarian are related but there is almost no correlation with English except for, say, the days of the week in Estonian to English.
If you know one you know them all? Not really, but it should be a little easier to learn a language from your own group.