The prominence of Welsh in The Crown's new series is "incredibly useful" for the promotion of the language, a veteran campaigner has said.
The latest series puts Welsh at the forefront of the Prince of Wales' time in Aberystwyth and Caernarfon ahead of his investiture in 1969.
It also includes an episode focusing on the Queen's reaction to the Aberfan disaster.
The Crown is one of Netflix's most successful series.
"Millions of people are going to be aware of the existence of the language, as a living language, for the first time ever," said Dafydd Iwan.
Episode six, which covers the investiture, is called Tywysog Cymru - Welsh for Prince of Wales - and the Prince, portrayed by actor Josh O'Connor, is seen travelling to Aberystwyth at the request of the Queen to learn the language before the big ceremony.
On arrival at the university he meets his tutor Dr Tedi Millward, played by actor Mark Lewis Jones. Nia Roberts plays the role of Dr Millward's wife, Silvia Millward.
The husband and wife are shown speaking naturally in Welsh in scenes at their home, with English subtitles on screen.
Jones said the Prince and Dr Millward developed "a respect for each other during this time even though they came from different worlds".
He said having Welsh spoken in the series "means a massive amount".
"This is so important, I think, that this language is heard throughout the world, with subtitles and no apology for that," he said.
Roberts said she hoped "people will be pleased about how much Welsh there is in the episode".
"I don't say anything in English in this episode and I think that was a really brave decision by The Crown producers," she added.
Mr Iwan said: "I didn't feel uncomfortable at all about the way they treated the Welsh language. Unfortunately, we have become used to an English treatment of the Welsh language - which has often been done in either a patronising or ignorant fashion."
He said the episode gave a "fairly fair treatment of the language as a living language".
The Crown producers approached him to ask permission to use his song, Carlo, and to collect "ephemera and posters" for the episode.
Posters of language pressure group Cymdeithas yr Iaith Gymraeg and one advertising Mr Iwan can be seen in the background of some scenes, something he admitted was "a little strange".
"They work within a historical, factual framework, and then use a bit of license to use their imagination," he said.
While the historical opposition to the Prince's presence is shown as Charles arrives in Aberystwyth, and as his coach travels through Caernarfon on its way to the castle, Mr Iwan said he had expected to see more. But he added that Dr Millward's character did convey the dissatisfaction effectively.
"I would expect to see more of the conflict, and crowds protesting against the investiture. But they chose to focus on the language, which is interesting and a good thing for the Welsh language."
Mr Iwan said he liked the series, which was released on Netflix on November 17.
"They could have focused on the great success of the investiture and portrayed the opposition as some kind of crazy bunch of students. But by portraying the other side in the character of Tedi Millward they have given it a bit more strength.
"In other words, it could have been a lot worse."
In real life the language campaigner met Prince Charles earlier this year.
"It is true to say that he was very, very aware of the Welsh language and the importance of the Welsh language, and the aspirations of Welsh nationalists," he said.