As a double major if you do reasonably well in only the second major will it affect your chances of getting into a good grad school? I started out as an economics major after being pushed into it by my family. However, I never had much of an interest in this subject. I was going through a very rough time in the first two years and I never excelled much in economics and I did not care about what was happening to me. However, a math professor who is my current thesis supervisor encouraged me to look into the math department and that's when I thought about finally majoring in a subject I actually cared about and doing something for myself. Thus, I ended up double majoring in mathematics and I started finally feeling somewhat invested into my own future. I have done quite well in my second major (mathematics) and I hope to do a master's in either mathematics or statistics in France or Germany.
However, I am worried if I can even get into a good grad school as my grades in economics are not very good.

I have no idea how grad school admissions look into double majors. How will my case be considered if I want to do a master's in mathematics? Will my performance in economics adversely affect my chances of getting into a good grad school?
Is it recommended that I retake some economics courses I haven't done well in? I really don't want to. I would prefer to use that time to study more mathematics and preparing for grad school which is a cumbersome process in itself and retaking would also mean more finances behind a subject I don't care about. But if it comes down to that then I will have to give it a try.
If your second major (economics) is not related to the graduate program (pure? math), your performance in the second major will be viewed the same way as your performance in elective courses.  It is not that important but it will count towards any GPA cutoff.
If you were applying for a graduate program in economics with an intent to do quantitative research, bad math grades would be more important because math is used in some economics.
I would not recommend retaking a specialized course you do not plan to use for anything unless you would enjoy it.  It would be more efficient to take a course that is useful and also increases your GPA.