I think you may have been misinformed. Abba the group are all Swedish and Swedes are well known to have a very good command of the English language. Also their songs (the lyrics) are written in standard (English collocations). All the best long lasting songs written in English are written in everyday collocations, as well as having a melody and a tune combined with a catchy "hook". Although as a general rule you should not rely entirely on using English pop songs to learn English. Many are just rhyming nonsense.

The only criticism you could possibly make would be that they do not have a 100% native accent. Only a few subtle clues would give it away. Of course, because we know that they are Swedish, we might be tempted to tune in to these differences more than otherwise. Their lyrics are generally in full sentences and the meaning is clear. I think you would be hard-pushed to find a grammar or usage mistake in their lyrics. From these perspectives, their language level is much higher than that of lot of native-speaker artists.

I like them too. I think their English is usually very good, both in terms of grammar and word choice. I think it would be fine to use their songs and interviews to learn English if that helps you stay motivated. Of course, they do have a Swedish accent, so you have to be aware their pronunciation is not the same as a native English speaker, but their accent is easily understandable.

English (Ancient English) evolved from the shores of the the north Netherlands = the Frisian coast or MODERN Friesland. An area which is a large part of the Netherlands and a small part of Northern Germany. A modern English speaker would be able to catch the gist of some of todays modern language spoken in these FRISIAN areaS. They would also be able to follow the rhythm of the language. Which might explain why they speak good English in these areas. When I flew to Amsterdam the passengers were speaking Dutch on the plane and i could pick out little bits and fill in the gaps. And at the airport I could still catch the gist of the Dutch airport announcements.

Generally, people from the nordic countries (Denmark, Norway, Sweden), and also the Netherlands, speak English pretty well, better than most other European countries.