Having worked in many EU countries, I've never once had salary quoted or discussed after tax.
At the end of the day it doesn't really matter if it's an employer tax or an employee tax, the employer spends some amount of money and the employee gets a portion home. But the discussion never includes employer taxes.
Either way, the point stands - for cross-border salary discussions, you need to consider the “fully loaded” cost of an employee, not just net salary. You also have to consider non-cash benefits and costs (high US salaries offset by high health care costs, etc).
I'm sure there's some shuffling they would end up doing when it comes to wages, stock options (not really a thing), transportation, and other benefits.
E.g. My employer pays X1.25 for my "before tax" salary of X because there is 25% payroll taxes/fees. I pay 35% taxes on that, so my net is 0.65x. Is X "before"or "after" taxes here? It's after payroll taxes and similar deductions, but before income taxes (which I pay myself).
Officially, on the employment contract, the salary is shown before-tax, but after mandatory pension and healthcare deductions.
However, in the negotiations, in casual conversation, and when you're hearing someone talking about their salary in the media, it's always discussed after taxes. Most people have no idea how much tax (or health insurance) they're actually paying.
And if you're working from another country, then you're a fiscal resident in that country and you have to pay taxes there. But in that case you'd be a contractor or own a company, so that's a totally different story.