Every page in the main guide looks at the destination side — can you naturalise in Germany, France, Spain, keeping your original passport? That’s only half the question. The other half is whether your origin country lets you keep its citizenship once you do. The answers range from a one-page oath to an automatic, unappealable loss — and for one country on this list, the rule is still genuinely being written.
These rules describe the origin-country side only. Pair any of these with how destination countries handle your original citizenship — both sides have to agree for you to safely end up with two passports.