Currency devaluation happens automatically regardless of government intervention. Where did you get the idea that it's just a currency policy? Currency devaluation and appreciation are purely market driven depending on supply and demand. If you have two countries trading with each other and one of them has a surplus like the trade between Germany and Greece. Greece has to borrow money from Germany for a surplus to happen. The market automatically compensates for this. What happens is that the demand for the German currency shoots up and demand for the Greek currency goes down. This means that eventually German products are no longer affordable because the Greek currency has been devalued and Germany won't lend money to Greece anymore because they can't pay it back. This also means that suddenly Greek labor is much cheaper than German labor despite the productivity gap. Therefore there is now an incentive to start more Greek businesses.

Anyway the takeaway is that there is an upper bound to how much Greece can borrow from Germany before local industry becomes more cost effective. The goal is to trade as much as possible but to also support the local economy.

Now the same scenario with the Euro results in no appreciation or devaluation. Greece borrows money from Germany. The exchange rate doesn't change because it's the same currency. Greece's ability to repay the loan did not change but there is now no economic pressure to not lend money. Greece can lend as much as it wants and since German products are always more cost effective it will use that borrowed money to always buy German products. The local economy languishes and eventually Greece has borrowed so much that it can't pay off it's debt. Then all you need to do to finish off Greece is to launch some kind of austerity policy that decimates what little local economy has managed to survive.

There is no working solution for the problems a unified currency causes. The best thing that one could possibly do is to have transfer payments to cancel the surplus but there is no way to force people to spend that money in the local economy and there is still the perverse incentive to increase the surplus to get more transfer payments.