But for example, if the main client is a tourist from England to Germany, wich serp do I have to focus on?, google.de or google.uk?
By default, you get Google.com used for most browsers, including chrome, for both mobile and computer,
And language will be set depending on the user's current location,
It really depends on the user's set preferences and the browser he's currently using as well,
So, if the user is setting up English as a default language, it will remain the same,
Not to mention that he might still get the current region-specific websites ranking even in a foreign language for the English SERPs,
The case depends here, and there's no set rule for this,
Make your website extension based on the country you're targeting, DE for your example,
Then, depending on the keyword research, you can target the one with more volume or even both languages