In Spain, "acciones" are shares/stock in corporations/public limited companies ("sociedades anónimas"), while "participaciones" are shares/stock in limited liability companies/private limited companies (sociedades de responsabilidad limitada). Since as you mention, by law Spanish limited liability companies members' interests cannot be called "shares/stock", their "participaciones" must be translated using another term. Some of the ones most often used are “equity interests,” “owner (or) member interests/holdings/quotas,” etc. And sometimes Spanish limited liability companies themselves refer to their “participaciones” literally “participations,” “participation units” or “equity participations.”

“Shares” and “stock” are synonyms for “acciones,” so I don’t think these terms convey the meaning of “participaciones.” There has been much discussion on Proz concerning this idiosyncrasy of Spanish corporate law, so you might want to take a look at previous KudoZ questions concerning “participaciones”.

But perhaps there is no single “right” answer here, other than choosing a term that underscores that “participaciones” are not “shares/stock,” a translation that should probably be reserved for the “acciones” of “sociedades anónimas” rather than the “participaciones” of “sociedades (de responsabilidad) limitada.”



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Note added at 57 mins (2018-07-20 09:20:05 GMT)
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For an additional comment, it is often assumed that “shares” and “shareholder” are the British English terms for "acciones" and "accionista," while the American English equivalents are “stock” and “stockholder.” But this is not actually the case. Although "stock" and "stockholder" are the terms used in the well-known Delaware General Corporation Law, “share” and “shareholder” are the terms used in the American Bar Association’s Model Business Corporation Act (MBCA) and its revised version (RMBCA) adopted in whole or in part in over half of the fifty US states.

But, to clarify:

"Both the terms stockholder and shareholder refer to the owner of shares in a company, which means that they are part-owners of a business. Thus, both terms mean the same thing, and you can use either one when referring to company ownership."
https://www.accountingtools.com/articles/what-is-the-differe...


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Note added at 2 hrs (2018-07-20 11:00:28 GMT)
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I see that Patricia has provided a link to one of several of my previous KudoZcontributions explaining the difference between "acciones" and "participaciones":

https://www.proz.com/kudoz/spanish_to_english/law_general/31...

Other possible translations are discussed there.