No reason not to translate "a la sal", in my opinion, and this way people with no knowledge of Spanish know what they're getting. The technique of oven-cooking "a la sal", where you lay the food (often fish, but can be other things like potatoes) on a bed of salt and then cover it over with more salt, is called salt-baking in English. If you do it in a bag it amounts to the same thing; the food is completely enclosed in salt.

"Salt-baked potatoes" is a fairly well-known dish:
https://www.google.es/search?num=100&q="salt baked potatoes"...

"Salt-crusted" is similar, but there you coat the potatoes in salt rather than completely burying them.

--------------------------------------------------
Note added at 23 mins (2014-01-19 17:49:12 GMT)
--------------------------------------------------

I'm all in favour of leaving the original name of the dish, papas a la sal, and adding a translation or explanation, but I think the translation or explanation is needed, and I think it can be "salt-baked potatoes". If you prefer, it could be "potatoes baked in salt", but as I've said, "salt-baked" is standard in English.

--------------------------------------------------
Note added at 40 mins (2014-01-19 18:05:48 GMT)
--------------------------------------------------

"A la sal" is the Spanish term for this technique, but it is not specifically Spanish. It's traditional in many Mediterranean countries, and has been used in China for 2000 years. So I don't think there's a special reason to preserve the Spanish term in this particular dish.
http://www.washingtonpost.com/lifestyle/food/salt-baking-veg...