I don't know the French situation, but for Germany: If you are an arbitrary employee with only a salary and maybe a savings account you don't have to do anythin

I don't know the French situation, but for Germany: If you are an arbitrary employee with only a salary and maybe a savings account you don't have to do anything. The employer will file salary taxes, the bank will file taxes on interests. At the end of the year they send summaries and that can be it. As soon as you have income from other sources (i.e. self-employed work etc.), want to deduce more than 1000€ etc. you have to get into the paperwork and then it can become quite complex if you want to really take all benefits in.