Germany: no ratio at all, a household budget instead
Germany publishes no debt-to-income rule, and that is a finding rather than a gap. The Wohnimmobilienkreditrichtlinie requires a lender to assess creditworthiness, but it sets no number, so there is no statutory ratio to pin and none is claimed here. The 35% on this page is a market convention: it is roughly where German lenders want the mortgage payment to sit relative to net income, and it is labelled as such.
What a German bank actually does is a Haushaltsrechnung, a household budget. Net income in, every recurring cost out, food, insurance, transport, existing credit, an allowance per person in the household, and the mortgage payment has to fit in what remains with room to spare. That is a stricter test than a percentage for a household with children or high fixed costs, and a looser one for a frugal high earner, which is exactly why no single ratio was ever published.
Income is assessed after tax, so enter your take-home pay. The German rule of thumb sits below the American 36% not because Germans borrow more cautiously in some abstract sense but because the base is smaller: 35% of net is a far tighter constraint than 36% of gross.
The other German particular is the deposit. Buying costs are substantial and are generally not financed, so a real amount of equity is expected up front, and it is frequently the deposit rather than the ratio that decides when a purchase is possible.