As mouviciel explained, you can indeed combine stamps to reach the required value. You can do that even with stamps that don't have a value printed on them. They remain valid even if the prices changes and their value is the current price for the relevant service (green stamp = letter in France, red = priority letter in France, blue = EU, purple = rest of the world). You can also buy 1, 5, and 10 cents stamps to complement regular stamps.
A regular red stamps is currently worth 0.66 € and an international letter (outside the EU) costs 0.98 €. So two red stamps or even two green stamps are definitely enough for an international letter.