Not true at all! My (EU) tax form has a special field just for this case - income from out-of-EU employment... (and because of tax residency rules that applies

Not true at all! My (EU) tax form has a special field just for this case - income from out-of-EU employment... (and because of tax residency rules that applies only if I live there) And there definitely is a legal framework - this has very exact rules specified by the bilateral tax/trade agreements, every combination of western countries has them, and many non-western countries too.