If you are a business, or you accept Circles for some kind of goods or services you provide, you should pay taxes in Euros or in your local official currency. Just like you would for any other kind of income: bitcoin, euros, dollar, etc.
If you are a private person, it’s similar. If you sell your bike for CRC, you don’t need to pay taxes. However, if you sell a lot of bikes, even if you are not a business, you might have taxing obligations.
Taxation is relative to your jurisdiction and must be handled accordingly. We do not offer tax advice and recommend that you speak to an expert from your area.
Here are sample tax assessments of concrete economic consumptions of Circles, based on German laws.