It is very helpful to consider how you will spend your money in retirement. A lot of your expenses in retirement, especially services, are priced in Euro and paid for in Euro. However, many expenses depend on global prices like electronics and clothing, and as I'm sure you've noticed recently in Cyprus and the UAE, the strength or weakness of your currency can make a big difference in the price of those goods and on your quality of life.
If you were to go and just invest in a balanced international portfolio like the Vanguard Total World ETF you would have too much foreign currency exposure and if the Euro was particularly strong at retirement, the local goods and services might be relatively too expensive. However, if you hedge all your international portfolio back to Euro like base64 suggests, you could be in a tough spot buying your needed global goods if the Euro is particularly weak. While you could get lucky in either of these scenarios if the Euro moves the right way at the right time, it can be a dangerous thing to bet on.
Your expenses in retirement are almost certainly more balanced than either of the two extremes above, so your portfolio should be balanced as well. Luckily, there are two fairly simple ways to accomplish this that are generally available to a European investor. One is to have a globally diversified portfolio with a bias toward the local currency; while you lose some of the diversification benefits of a fully global portfolio, this is generally very easy to set up and maintain. Also, you could have a more global portfolio but hedge some of the currency risk to your local currency. The best way to do this these days is holding currency hedged ETFs as base64 suggests; just make sure it is only part of your international portfolio. However, this can add more costs to your portfolio.
While I can't give you what percentage in Euro is the best percentage. One thing to keep in mind though as you think about your retirement spending is that both goods and services have been getting more globally priced over time. If you think this will continue to when you retire, as I do, you may want to bias your portfolio to have more foreign currency exposure than someone that would be retiring sooner.
Edit: @base64 Fully-hedged etfs actually add currency risk when compared to Demos' retirement expenses which will not be 100% Euro denominated as he will need to buy a combination of global and local (Euro) goods. My best guess for a portfolio that makes not Demos' portfolio but more important Basement Jaxx's retirement agnostic to currency moves is a globally diversified portfolio either with a tilt toward European stocks or with a small amount of currency hedged international etfs.