The “thank you - please” pairing is interesting. The sentiment is always polite and, therefore, friendly.
The pairs I am aware of are
Welsh Diolch - Croeso (I’ve also heard dim o gwbl)
French Merci - je vous/t’en prie ou “de rien” ou “pas de quoi”
Italian Grazie - prego
Spanish Gracias - de nada ?
Dutch dank u - alstublieft
British English Thank you - don’t mention it or (more recently) you’re welcome
Australian English - no worries 
American English - you’re welcome or just mmm-hmmm
German needs thought Danke - bitte; BUT danke schoen - bitte schoen.
In the 60s, when I thanked a German gentleman for his extravagant hospitality to me and my freundin while youth-hostelling, i think he said something like “Da fuer nichts” which I took to mean "it’s nothing.
I can now add @tatjana 's pair to my list
Slovenian Prosim - Hvala or Prosim - Ni za kaj. (what would be something like it’s nothing) We also say Še drugič. (what would be like Another time yet.
Young people many times to Hvala (thank you) reply among themselves as OK, ni panike. what would mean OK, no panic. But this aplies to really good young friends.
In Serbo-Croat mixture of languages we were taught in ex Yugoslavia to word Hvala (thank you) the usual reply was Nije na čemu (what would be like it’s nothing) So this can go to your list too @hewrop as I can speak prety good Serbo-Croatian too. 
Enjoy doing this list. With more expressions it will become really interesting in time. 
I heard a new English word on Radio 4 of the BBC, no less, today.
If English is influencing the way Welsh develops you certainly need to prevent the introduction of this word. It was ‘actionise’. Don’t ask me what it means as I don’t know. I looked it up in the dictionary and it wasn’t there. Ah, well, it appears that I have been left behind but if this is the future I am, actually, not sorry.
A while back, it seemed to become the vogue (in the UK, possibly from US influence), to reply to
“Thank you” - with “No problem”. Harmless enough I suppose, but I never warmed to it.
Much more American, and hopefully going out of fashion would be:
“Thank you” to which the (almost) reply was: “Sure!”.
I can add some more American responses to “thank you” from different regions I’ve lived in.
Midwest:
My pleasure!
You’re welcome.
West Coast Cali:
Of course!
No worries.
Sure.
Sure thing.
Yup.
Not a problem/no problem.
I rarely hear “you’re welcome” as a response except in formal situations and sometimes with strangers. It’s also very common to hear the Spanish “gracias-de nada/no problema” combination in otherwise fully English conversations here in SF for people under 50.
Cultural differences in how you use these courtesy words are interesting too.
In English you can nearly always respond to a compliment with Thank you and it feels quite natural.
In Italy, if someone says, for example about a meal you just made, ‘‘that tasted great’’ you can reply with ‘‘Grazie’’ but that will force an almost obligatory reply back of ‘‘prego’’ which feels a bit unatural… So it is more customary to reply to a compliment with a returned compliment - for example, ‘‘yes, but I wish I could make Tiramisu that tastes as good as yours.’’
That avoids the obligatory ‘‘prego’’ to your ‘‘grazie’’
A subtle and useful point, J & E. I must remember that.
Oh dear, when will people (usually politicians or their spin doctors) stop ruining languages by inventing ugly ways of saying simple things? I show my age by getting cross about the alteration of meanings as well!! “Arguably” should mean “uncertain/open to argument” and is getting used on the BBC to mean something close to certain!!! The complete opposite in fact!!!
And let’s not even talk about “alibi” - a lost battle, and the loss of a useful differentiated meaning.
When I was struggling to learn Danish, I discovered that the Danes have an interesting custom at the end of a meal in which the hostess takes the initiative and declares the meal over (I forget the form of words, even in English), and this then is the cue for the guests to thank (traditionally) her, and of course offer compliments.
I think we have already mentioned the (to me) charming Danish custom of whenever people have spent time together (often at work, but doesn’t have to be), they say to each other “thanks for today” - “tak for iday” (I think). Not sure if other Scandinavians do something similar. I noticed this a few times on “Borgen”.
Basically, look you, its been a whole fortnight since I was bad in bed under the doctor.
Yes, “De nada” is a good Spanish response; a sort of “It’s nothing.” “Alstublieft” (or more informally “Alsjeblieft”) tends to be said (at least in Belgian Dutch usage) when offering or giving something. The polite response to a “Dank u/je” is “Niets te danken,” or “Graag gedaan.”
Dank u wel, David (you can skip the polite response 
)
On the original subject of Wenglish, did anyone watch the S4C programme about the Welsh Guards? In between fuming about “the officers are all English”, I was aware that I hadn’t heard so much Wenglish since I moved to Scotland!!! from Jackie
So you could definatelly understand everything …
I think that even the ever-so-English officers could have understood everything!!!
p.s. I’m still fuming at the fact that the officers are English!!
I’ve listened to Radio Cymru yesterday afternoon on my phone for quite a while and what I was surprised with was that there were many pure English words in Cymraeg conversation not only by the guest in the studio (it was one artist who I didn’t remember the name of) but also from the presenter. And what surprised me even more - not even one number was said in Cymraeg, especially not years (like 1989 for example). Is this usual usage of the numbers these days or it was just the guest who didn’t somehow want or know to use Cymraeg numbers instead of English ones. He spoke Cymraeg very well (to my ears of course) and that’s why it all seamed even more strange to me.
The number thing is quite common.
However it all sounded prety odd even to my ears not used to hear Cymraeg conversation very often.
Thank you for reply.
In Ceredigion (and many other places) monetary transactions especially are conducted in English - not sure why, though.
You may also be interest to learn that the Welsh number system has been simplified in recent decades. It used to have some intriguing oddities (which I rather miss).
The number 16, for instance was effectively 1 + 15 and 18 was 2 nines. 40 and 60 were roughly 2 twenties and 3 twenties respectively. I deliberately didn’t give the old numbers to avoid confusion. 
Some oldies like me still use the old system when we are talking about ages and dates.
It’s conventional to use the ‘old’ numbers in ages and dates, though it’s becoming less common amongst young people. So it’s not just “some oldies”, Huw. You’re not as alone as you think!
I really like the characterful old numbers and enjoy winding up my daughter by using them in circumstances where she wouldn’t think them appropriate. But I’m not sure that I’d like to use them for complex maths.