There is by definition almost always a party holding a relative majority (more seats than any other party), which the CDU did for the longest time.

You are correct that they did not hold an absolute majority (more seats than everybody else combined), ensuring that they always had to form a coalition to achieve that.

I've never heard of the word "majority" meaning "relative majority" without that qualifier, but I also wouldn't use the phrase "relative majority" to refer to what to me is clearly a minority, so what do I know :)

Nevertheless, it might be better to use unambiguous terms like "plurality", or define ones terms, when writing for an international audience.