It follows a damning report which criticised Wrexham Council for failing to correct translation errors in council tax bills, despite them being brought to its attention by a resident in 2014.
The Public Services Ombudsman for Wales ordered the council to correct the errors in 2017 and told it to pay £50 in compensation to the man, known only as Mr D.
However, when he received his council tax bill in March, there were still inconsistencies between the Welsh and English versions of the document.
He said it was a sign of “disrespect to taxpayers, the Welsh language, the law and the Ombudsman”.
Leading councillor Hugh Jones admitted the authority had made repeated mistakes and pledged to resolve the issue.
Speaking at an executive board meeting, he said: “I have to start with a public apology, firstly to Mr D and secondly to the Ombudsman.
“The council failed to keep our promises to both of them.
“The report of the Ombudsman also notes that we have failed to uphold our Welsh language standards.
“On behalf of the council I apologise.
"I hope those who are responsible for keeping to our statutory and moral duties will learn a lesson from this experience.
“I can assure the council and Welsh speakers that we fully support the Welsh language.”
The Ombudsman has asked the authority to write a further letter to Mr D and pay him £100, as well as asking for a system to be put in place to ensure it does not happen again.
Cllr Jones said a large amount of translation was being undertaken to make sure all council documents issued in Welsh were accurate, including benefits letters.
Council leader Mark Pritchard added the work was being carried out by Conwy Council.
However Plaid Cymru leader Marc Jones queried why the corrections needed were so complex.
The Public Services Ombudsman for Wales ordered the council to correct the errors in 2017 and told it to pay £50 in compensation to the man, known only as Mr D.
However, when he received his council tax bill in March, there were still inconsistencies between the Welsh and English versions of the document.
He said it was a sign of “disrespect to taxpayers, the Welsh language, the law and the Ombudsman”.
Leading councillor Hugh Jones admitted the authority had made repeated mistakes and pledged to resolve the issue.
Speaking at an executive board meeting, he said: “I have to start with a public apology, firstly to Mr D and secondly to the Ombudsman.
“The council failed to keep our promises to both of them.
“The report of the Ombudsman also notes that we have failed to uphold our Welsh language standards.
“On behalf of the council I apologise.
"I hope those who are responsible for keeping to our statutory and moral duties will learn a lesson from this experience.
“I can assure the council and Welsh speakers that we fully support the Welsh language.”
The Ombudsman has asked the authority to write a further letter to Mr D and pay him £100, as well as asking for a system to be put in place to ensure it does not happen again.
Cllr Jones said a large amount of translation was being undertaken to make sure all council documents issued in Welsh were accurate, including benefits letters.
Council leader Mark Pritchard added the work was being carried out by Conwy Council.
However Plaid Cymru leader Marc Jones queried why the corrections needed were so complex.
