Assuming you mean for the EU.

That would involve many 26 other countries having to learn German, so they could talk to each other, in the way they currently do with English.  Two of those EU countries, Ireland and Malta have English as their official language.

Those 26 countries, plus Germany, would also need to learn English to communicate with countries outside the EU: United States, United Kingdom, Australia, New Zealand etc.

Sometimes things are best left as is.