Unfortunately, late payments are common nowadays. If a payment is not made within the agreed time limit, it is advisable to remind the other party of the debt first, for instance by sending an informal letter and later even a formal reminder. If the debtor does not respond to the letter or reminder within a reasonable time, recovering the debt in court should be considered. However, an unwritten rule says that debts up to approximately CZK 20,000 (i.e. about EUR 740) are mostly not worth enforcing in court because of the costs and length of such court proceedings. This is especially true in cross-border matters (debts).
If the date of payment is set firmly in the contract, which is the common case, the other party gets into default by the very lapse of such date, so no reminder or request is necessary. If a reminder is made in such case, it is merely a manifestation of good will toward the other party.
If you decide to recover your debt in court, you can initiate legal proceedings by taking a legal action (ideally along with filing an application for a payment order) directly against debtor. Such procedure can be applied not only against a debtor with a registered office or residence in the Czech Republic but also against a debtor based in another country, i.e. at the international level.