On paper, paying the higher-interest debt back first with the entire bonus amount is the most straightforward solution. However, many people will tell you to establish an emergency fund with some of the money. This may make sense for you, but (like pretty much all financial decisions) it boils down to risk tolerance. What are the chances that you will have a financial emergency in the near future? If you are nervous about that risk, set some money aside now. But keep in mind that doing so means you are paying more interest on the debt you owe. So - it's a tradeoff - by establishing the emergency fund now you pay slightly more over time in order to have a hedge against risk. Some people will be in a stable enough life situation where an emergency fund isn't a high priority (i.e. you have good health insurance, live with your parents, walk to work, and don't own anything like a car or house that could incur unexpected expenses). Those people may choose not to save for an emergency fund. Other people may be in riskier situations (own a home, own a car that they need in order to get to work, contract job, family to support, etc) and for those people, paying a little extra interest on the debt will be an easy trade off for some security.
Also, since you mention that you can save each month, it's worth considering that the 400 euro you can potentially save in a given month is just a mini version of this same decision. Unless you have some tax-advantaged retirement plan option available, it usually doesn't make sense to budget for long-term savings (i.e. anything beyond an emergency fund) while you still have unsecured high interest debt.
All that said, your immediate plan should be something along the lines of:
- Decide if you want an emergency fund, and how much. Set aside from your bonus or monthly savings until you reach that amount.
- Pay the rest of your bonus or monthly savings towards your highest debt, until those two loans are paid off.
- Plan for long term savings and contribute from your 400 a month towards that as appropriate.