An apparent ban on staff speaking any language but English in a Sports Direct store in Bangor is to be investigated by the Welsh language commissioner.
It is understood a notice was posted on a wall telling staff to only speak English for health and safety reasons.
The move has also drawn criticism from Plaid Cymru AM Sian Gwenllian who said it was "discriminatory" in an area where a lot of people speak Welsh.
But the firm said there was no ban and it was looking into what had happened.
The notice, which was printed on Sports Direct-headed paper, said it wanted to remind staff that English was the "official language of the company".
"It has come to our attention that some members of staff are speaking to each other in languages other than English whilst carrying out their duties," it said.
"We would like to take this opportunity to remind staff that they must speak in English at all times when they are at work, in order that they can be understood by all members of staff."
It added: "Staff speaking in languages other than English can pose a variety of risks to the company, including health and safety issues arising out of not being understood by those around them."
Welsh Language Commissioner Meri Huws said she had instructed her office to investigate whether the company had broken any rules under the Welsh language measure of 2011, which made it illegal for companies to stop people speaking Welsh to one another.
Ms Gwenllian, who represents Arfon, called for an apology from the company.
"In an area such as Bangor where there is a high number of Welsh speakers, it is highly likely that such a policy would deny staff members the right to converse in their own language," she said.
"I will be submitting a formal complaint to the owner of Sports Direct."
Sports Direct insisted the ban was not the result of a directive from the company.
A spokesman said an investigation was under way into the notice, including its origin, authenticity and context.
"Sports Direct is an international business, operating in many different jurisdictions. We encourage the use of the native language and would never instruct our staff otherwise," he said.
"It is not company policy to restrict the use of the Welsh language or that of any other country."