Generally, when the Balancing Statement says that the underpayment will not be collected at this time, it means that it will not be collected at all unless/until you are due a refund in which case they deduct this amount of the underpayment from the refund due to you. They seem to operate a threshold whereby if an underpayment is under that amount, they will not seek to collect it. I don't believe that they will reduce your tax credits as they would have clearly stated this if this was their intention.
To save you the expense of getting an official translation done, I would send them the foreign tax statement with the tax figure highlighted. You can easily translate the foreign word for tax on the web. It has worked for me previously.