I also have had to deal with this in my sites.  In most cases, I actually go with the norm that the language is based in.  By this I mean that if the page is in English, even if I am using a Spanish or French word, I tend not to use the accent.  This is primarily because based on my statistics - I have been doing multi-language SEO for my sites for over 5 years - most people do not have their keyboards configured to make accents.
   So, if somebody was looking for information on:

Mexico

It is much much more likely that they would put that in google instead of:

México

Most non-tech people would struggle to figure out how to put an é, ñ or a ö on U.S. keyboards.

Of course there are some places on your site that you need to put the accents, especially if it is something like language education where they expect it to be with an accent, but otherwise I do not put them.  Of course, if you have your entire page translated into Spanish or French, then put the accents in the correct places because there they will be expected and the searchers will probably look using accents.  As you probably know, an accent in Spanish, French, etc can change the entire meaning of a word.

I hope this helps.

P.S. Hola desde Querétaro.