What do I do if my French customer has not settled my outstanding invoice ?
In general terms, an initial check needs to be carried out to ensure that there are clearly agreed payment terms between the parties, notably as to payment date and method.
Thereafter, and as an example solely relating to failure to comply with payment terms, a "mise en demeure" or formal demand should be addressed to the debtor by registered post with acknowledgement of receipt slip.
If this does not produce the required result, it will normally constitute the first piece of documentary evidence should Court action be necessary at a later stage.
What do I do if my French customer has gone into receivership/ liquidation (i.e. subject to statutory insolvency provisions)?
See FAQ May I reserve title to property which has not been paid for ? – but generally unless some sort of security has been obtained prior to the insolvency the only route would be to enter a formal claim with the insolvency practitioner as an unsecured creditor, and it should be noted that entering a claim has to be carried out in a specific manner.