Can anyone offer a translation of this?

 

How about "Is the Pope a Catholic"?

Beggars can't be choosers?

longdo.com has this:  จะให้อะไรแก่ผู้ที่เต็มใจรับอยู่แล้ว ไม่ควรถาม

 

So, there doesn't seem to be an English equivalent expression, but it would be in the vicinity of "carrying coals to Newcastle."

It depends on how you interpret it.  Giving alms is a duty, the monk needs it to live, so it is wrong to ask if he want it.  The explanation doesn't refer to religion. 
If you are going to give something to someone who will be happy to receive it (needs it) don't ask them if they want it.  If you ask a Thai, shall I pay for this meal they don't say yes please, they leave it "up to you". 
 

Westerners tend to not know this principle which may explain the expression พรั่งขี้เหนี่ยว

3 hours ago, tgeezer said:

It depends on how you interpret it.  Giving alms is a duty, the monk needs it to live, so it is wrong to ask if he want it.  The explanation doesn't refer to religion. 
If you are going to give something to someone who will be happy to receive it (needs it) don't ask them if they want it.  If you ask a Thai, shall I pay for this meal they don't say yes please, they leave it "up to you". 
 

Westerners tend to not know this principle which may explain the expression พรั่งขี้เหนี่ยว

I also think it can mean there's no need to ask, when the answer is obvious.

 

I used it with a friend, He asked if was wanting to have a game of cards next week and I said ตักบาตรอย่าถามพระ he thought it was very funny and said it was in context.

1 hour ago, Neeranam said:

I also think it can mean there's no need to ask, when the answer is obvious.

 

I used it with a friend, He asked if was wanting to have a game of cards next week and I said ตักบาตรอย่าถามพระ he thought it was very funny and said it was in context.

Yes, people see things in different ways which I related somewhere, all the posts have merged in my mind now.  
I have only the Thai dictionary for reference.  So if you and your friend see " Is the Pope a Catholic" in ตักบาทร ย่าถามพระ  so be it. 

I must have posted my correction of ขี้เหนียว under another topic heading, so  since the error ขี้เหนี่ยว is in these comments,  I post again.

I think that I have to be a little less precise, I can see how Coals to Newcastle, Is the Pope a Catholic and variations on that are alright. 
I put too much emphasis on the original saying and the cultural aspect. I think that เต็มใจ speaks for itself but ไม่ควรถาม can be interpreted more freely.   Longdo says "should", and I don't think that it means the same to everybody. Rather than "Don't ask because the answer is obvious" I see it as "Don't ask because it would be wrong to do so."  I have learnt that พระ is a loose term for a monk but I don't think that it diminishes their status.