Why have a will in Spain if I already have a will in Germany? Quite simply to clearly regulate your inheritance and to avoid possible interpretations of the German will by Spanish authorities. In addition, you are doing the heirs a favour, as by submitting a Spanish will, other documents to be applied for in the testator's country of origin (often expensive and cumbersome) become unnecessary.
First of all, you can only choose the substantive law, i.e. the possibilities of injunctions. The obligation to pay tax always remains subject to Spanish tax law. If you are not a resident, the choice of law is quite simple, the law of the country of your habitual residence is usually applied. Nevertheless, this point should be mentioned in the will.
If you are a resident in Spain but have a different nationality, you can now choose which of the two laws you wish to use. What is better in each case or meets your needs should be clarified in a consultation.