As stated above, I think language skills are something that matter, particularly given that latinos are one of the most medically underserved populations in the United States.
The definition of URM was intentionally broadened to encourage schools to look beyond traditional URM-designated groups to the communities within their own regions that are medically underserved and underrepresented within medicine. Many hispanic populations besides Mexicans and Puerto-Ricans fit this expanded definition and schools frequently include them in their diversity initiatives. Being Dominican, for instance, will likely be viewed favorably if you were applying to Colombia (or another NYC school) or a school in Boston. Many California schools already adopt expanded definitions of URM that included certain SE Asian populations. As other central/south american populations continue to increase, I imagine schools will look favorably upon applicants from these communities as well (e.g Ecuadorian, as noted above, Honduran, Guatemalan, Salvadorian). People within these communities are also more likely to come from immigrant/low SES/disadvantaged educational backgrounds, which are all factors that contribute to a holistic evaluation of an application.
Argentinians are a unique case though. Argentinians and Argentinian-Americans, in particular, are demographically and socioeconomically distinct from most other Central/South American populations.. When talking about URM, which is looking at whether a community is underrepresented
in the US healthcare system or is medically underserved
in the US, Argentinian-immigrants are pretty dissimilar as a population to almost all other latino/hispanic populations in the US, and certainly the ones typically considered URM. Argentina was one of the richest countries in the world for the past century (until the recent crisis). It also experienced a huge influx of fairly well off European emigrants, including many Jewish immigrants (largest Jewish population in South America). This, among other things, led to Argentina being a fairly well off and decently well educated country. More importantly (for URM), the majority of Argentinians who immigrated to the US were in this educated class, including a large population of doctors, making Argentinians very overrepresented in the physicians workforce relative to their population in the US (ORM). This also means that the Argentinian-American population has much higher levels of SES than nearly all other hispanic/latino populations in the US. They are not medically underserved, economically disadvantaged, or URM as a class and quite dissimilar to the many latino populations in the US that are.
Doesn't mean an adcom won't appreciate the cultural and linguistic diversity an Argentinian applicant brings to a class.
Interesting Stats from 2015 Census. Argentinians have higher rates of educational attainment than White Americans and most other hispanic/latino groups:
Argentinians- 40.1% have Bachelor's Degree or Higher, 19.8% Graduate/Professional Degree, 11.2% Poverty Rate
White Americans- 31.9% Bachelors, 12.0% Graduate/Professional, 12.2% Poverty
Ecuadorians- 20.1% Bachelors, 6.0% Graduate/Professional, 17.2% Poverty
Puerto Ricans- 18.4% Bachelors, 6.1% Graduate/Professional, 24.6% Poverty
Dominicans-17.4% Bachelors, 4.6% Graduate/Professional 26.7% Poverty
Mexicans-10.8% Bachelors, 3.0% Graduate/Professional, 23.5% Poverty
Salvadorians- 9% Bachelors, 2.0% Graduate/Professional, 19.9% Poverty