Welll . . . I did learn a bit of Swedish (mostly via Danish), many years ago, so I know some of the things which can't be guessed easily, e.g. that 'et' or 'en' stuck on the end of a word is the definite article, or that god = good & gud (pronounced good, more or less) = god. Still, I couldn't have followed most of it without the subtitles.
When I went to work in the Netherlands, never having studied the language, I felt as you describe, that it sounded like garbled English. Sometimes snatches would be intelligible, but then it'd lapse into gibberish.
The original Swedish Wallander is on BBC 4 on Saturday nights, BTW. Series 2, 21.15. Repeated Thursday 22.00. I prefer it to the English version.