Is the head of state of the UK not allowed to speak a foreign language in the UK or abroad?

Of course they can speak foreign languages, Elizabeth II was fluent in French for starters.

https://www.youtube.com/watch?v=4L3_9BdgMXM

There's no prohibition on the UK head of state speaking in a foreign language, e.g. King Charles III spoke in Italian during his speech to the Italian parliament and at a state banquet on Tue 9th & Wed 10th April 2025.

https://www.bbc.com/news/videos/cx288j4egg2o
There's a transcript/translation here:
https://www.royal.uk/news-and-activity/2025-04-09/the-king-delivers-an-address-to-the-italian-parliament

So King Charles spoke Italian in Italy. Is he allowed to do so in England?

I can't find any reference to there being a ban on the monarch speaking in another language in their homeland. Queen Elizabeth used Latin in this speech - '1992 is not a year on which I shall look back with undiluted pleasure. In the words of one of my more sympathetic correspondents, it has turned out to be an "Annus Horribilis."', which is pretty foreign to many of us.

https://www.britannica.com/topic/annus-horribilis

The monarch welcomes heads of states from around the world and is likely to speak a few words of the visitor's language out of courtesy.

>> The monarch welcomes heads of states from around the world and is likely to speak a few words of the visitor's language out of courtesy.
e.g.
In July 2012, Queen Elizabeth II welcomed President Francois Hollande to England. She spoke to him in French:

Transcript
0:23 s a pas de jalousie à avir entre la
0:28 franceose ça c'est vrai que c'est très
0:32 troublant connu la sécheresse il y a
0:34 quelqu nous AV et je sais que vous aimez
0:37 beaucoup le cheval donc voilà
0:40 attentionci beaucoup et et pour vous
0:42 nous avons photos voilà comment va votre
0:46 Marie il va très bien

https://www.youtube.com/watch?v=LZR7pP7wzqQ

George I couldn't even speak English, his language was German. I am sure he did not keep quiet the whole time he was in England.

https://www.parliament.uk/about/living-heritage/evolutionofparliament/parliamentaryauthority/revolution/collections1/parliamentary-collections/parliamentary-collections-georgei/