Welsh-speaking patients are suffering and may even be put at risk if they cannot communicate with health professionals in their own tongue, the country’s language commissioner has warned.

Aled Roberts said young children brought up in Welsh-speaking households and bilingual adults who lose their grasp of English when they suffer conditions such as dementia are particularly vulnerable.

Welsh-speaking expectant mothers have also described how important it is for them to be able to communicate in their own language while giving birth, especially in difficult circumstances.

The Welsh language commissioner, Aled Roberts, said some NHS bosses had little idea of how many Welsh speakers they had. Photograph: Welsh Language Commissioner

Roberts is launching a campaign highlighting that speaking to a patient in Welsh if it is their first language is a need rather than a choice for many.

The number of Welsh speakers in the health service varies greatly across Wales. Some areas report that almost all staff can speak some Welsh while elsewhere it may be less than a fifth.

However, the commissioner is worried that some health bosses seem to have little idea of how many Welsh speakers they have. “This makes it very hard to ensure that Welsh-speaking patients are cared for by Welsh-speaking staff,” said Roberts. “The workforce planning has to happen.

“We cannot emphasise enough the difference it makes to a patient to be able to speak their first language when unwell, and that speaking Welsh is a necessity, not an option, for many patients,” he said.

Roberts said that if professionals could not speak to Welsh-speaking children in their own language but relied on parents to translate, safeguarding issues could be missed.

Glenda Roberts, who has dementia and visits a day centre on the Llŷn peninsula in north Wales, a Welsh-language stronghold, said she had noticed a decline in her second language, English. “I can see that my English is deteriorating. I’m going back to my first language,” she said.

Edwin Humphreys, a psychiatric nurse at the centre, said there had been cases when Welsh speakers were scored wrongly in dementia tests. “Recently a lady had been assessed through the medium of English with the doctor, and had not scored very well at all,” he said. “The doctor diagnosed her with very advanced dementia.

“But, through my conversations with the lady, I knew that she was much better than that, so we decided to conduct the same assessment through the medium of Welsh, and she scored higher.”

Humphreys, who is also a musician, said that when they played English-speaking artists such as Elvis Presley to some dementia sufferers there was little response. “But once you start playing Welsh songs, the faces immediately light up and a memory is triggered,” he said.

Heledd ap Gwynfor, from Carmarthen, south-west Wales, said it was hugely important for her to be able to speak Welsh when she was rushed to an operating theatre while giving birth to her son at Glangwili hospital.

“I shall never forget the young man to my left,” she said. “His job was to push a tube into my hand. He spoke Welsh, and this meant the world to me. I could feel myself relaxing more and more with him. I could express myself better to him than I could to the midwife.

“Unfortunately for me, the shift was ending for everybody in the ward during the birth, and a shift of new people were coming in. I remember the young man explaining to me that he had to leave and I was breaking my heart because I could really relax whilst speaking Welsh with him.”

Another Carmarthenshire woman, Eiry Miles, described her panic during a difficult birth. “The doctor scared me, as she said something about me being in mortal danger.

“However, the midwife spoke Welsh. She was fantastic in reassuring me, and explaining everything to me in a simple way that I could understand. Being able to have this discussion in my mother tongue was something very special.”

Tomos Owens, a GP from north Wales, said he spoke Welsh to about 90% of his patients. “Parents appreciated having someone who can speak Welsh with their child,” he said. “With children, if you speak a language they don’t understand, it’s harder to get them to trust you, and let you inspect them.”

Welsh speaker Siân Elin Williams, from south-west Wales, said her sister was taken to hospital in Cardiff 70 miles away with severe pneumonia that caused one lung to fail.

She said, “She was very young at the time, a girl from a Welsh-speaking home, and it was so important that she was able to tell the doctors how she felt in Welsh. She was away from home, but speaking Welsh made her feel much more comfortable than she would have felt if she had to speak an unfamiliar language.”

Manon Williams, who leads nursing teams on cancer wards in north Wales, said: “It is very important that we talk to patients and their families in Welsh. With cancer, the information that a patient receives can be very complex. It is also very important if a patient and family receive bad news – they need to be able to talk about it in Welsh.”