Individuals are taxed on their worldwide income

All resident individuals (natural persons) are taxed on their worldwide income.
Domestic law deems a person to be resident if he has a home or his habitual
abode in Germany. Generally, individuals are deemed to have their habitual
abode in Germany if they are physically present for more than six months in
any one calendar year or for a consecutive period of six months over a year-end.
However, persons with their permanent and habitual abode in Germany will
not lose their tax residence simply because they happen to be travelling for
longer than six months.

Non-resident individuals are taxed (usually by
withholding) on their German-source income only.