I was adult years old when I realized all those big walls of gibberish I saw in my youth was intended to be elaborate graphics, but my computer had the wrong charset to show them.
I would guess MS-DOS. Back then, there was no UTF-8, and the character encoding depended on your language. People using English normally were using CP437 (https://en.wikipedia.org/wiki/CP437), while people using other languages would be using something like CP850 (https://en.wikipedia.org/wiki/CP850). Take a look at the encoding tables in these two articles, and notice that CP437 has lots of line and box drawing characters in the high half, while CP850 replaces many of them with accented letters.
If the file was written on a system using CP437, and used these line/box drawing characters, then someone on a system using CP850 would see random letters where the author intended fancy boxes around their text.
(This was due to a limitation of the text modes used to run MS-DOS: each character on the 80x25 fixed-size grid shown on the screen was described in memory by a single byte which was a index into the font table, plus another byte for attributes like color and intensity. That means there could be at most 256 distinct characters, and no way to combine separate characters into one. To add all the accented letters necessary for many languages, something had to be removed; and what was removed were the less important line and box drawing characters. That is very different from the graphical modes common nowadays, which store the color of each pixel separately in memory, and allow infinite variation on the character shapes.)
> Unsure which code pages were used but this was in Sweden.
Interesting. Every PC I ever used in Finland (home, school, friend's, etc.) were always using https://en.wikipedia.org/wiki/Code_page_437 and I would have assumed every PC in Sweden did too. Maybe your did have cp850 or something uncommon.