Taxes are different in France and in Spain, and all the prices in every place will show you the price of what you're doing, tax-included. One can move freely between the countries, use the same currency, and settle down in either place (given you're in the EU). This works despite the languages being different, and the governments being different. It also happens to work in quite a few more countries too.

I think I'm missing something when people use the excuse of "taxes are different in different places" to say "we don't show taxes out of habit, or because it requires us to change our signs".