> Teach the users English may be an altruist goal in the long term, but
> for many teachers (like my case) it a barrier right now that can tip
> the balance to other "more friendly languages"

Students are not required to learn English and English grammar before programming -- learning a few words is already enough to grasp the meaning of many error messages (and it's anyway already necessary for keywords, functions, methods, classes, modules, etc.).

> Those that continues working on programming will surely be exposed 
> sooner or later to formal technical English course at University or 
> similar.

The sooner they get exposed to English the better it is.  The best way to learn a language is by using it, and IMHO technical English is even easier than "normal" English (and often even than native language).

> But, if they don't continue their studies, or choose a different 
> career, maybe their English skill will never be enough.

I think that nowadays anyone should learn English anyway, and the more you translate the more you make their lives difficult, because you confine them to a restricted subset of all the available information (this is getting off-topic though).

> Here, as you point, translation poses a new perspective, why take
> that as a threat instead of an opportunity to bring better messages?

This is a different problem though.  Python (and programming in general) has its own jargon, and the jargon provides a concise way to refer to specific concepts (e.g. tuple-unpacking).  While it certainly shouldn't be abused, it's often more convenient to use it.  Creating a new localized jargon also doesn't help, and it only makes things more complicated.

For example you mentioned the "invalid token" error message, that on the page you linked is translated as "token inválido".  AFAIK "token" is not a Spanish word, so either you end up leaving the jargon untranslated, or you translated in something that doesn't make much sense and doesn't match with other names (e.g. the tokenize module).
While improving the message is a good idea (if/when possible), translating it doesn't make things much better.

> localized error messages could eventually produce better search
> results for non-English speakers if there is enough material written
> in they language. 

IME experience it's the opposite.  This might work a bit better for widespread languages like Spanish, but otherwise I often come across to fairly bad results.  First of all localized documentations are not as updated as the official one (that gets updated daily), and the translation might not be accurate.  Given that the English community is the biggest one, it's also easier to find answers in English than it is in any other language.  People that are not using English resources are often inexperienced users, and the solutions they provide are not always good (of course there are exceptions).

> At least some part should be translated too, as for example, the 
> Python Tutorial was translated by the local community to Spanish:
> http://docs.python.org.ar/tutorial/contenido.html

But this is just a part, has not been updated in over 2 years, and doesn't even cover Python 3.