A few questions about the student visa

Hello, recently I�fve been considering enrolling into a language school in Japan. I should also mention that it will be my first time visiting/going there, that is if I eventually end up doing so. I�fve always been fond of the language and I would like to see how it is to live and study in the country.
I have a few questions though: who can be my sponsor for my studies? The only person with the ability to sponsor me would be my grandfather, who is also my legal guardian, yet he is actually retired. He does not have actual employment but rather retirement benefits (he does have the means to support me financially despite this though). Would this be a problem? Also, I�fm currently living in a country that is not my �ghome country�h or country of nationality. I�fm American but I�fm living and I�fve gone to high school in Mexico. Could this also be a potential problem in the application process, for the school and/or the visa? I know this is a pretty special case, and figuring it all out may be confusing, but I would very much like this goal of mine to become a reality, I would really appreciate your help!
Thank you in advance for your responses and I�fm looking forward to them > <

When people talk about a visa sponsor, they're talking about the person or organization in the overseas country who will vouch for the person. If you're applying for a student visa to live in Japan, the sponsor has to be a person or organization in Japan, so your grandfather would not be able to sponsor you (unless your grandfather is already living in Japan, which I'm guessing is not the case).

For a student visa, your sponsor would be the school you'll be attending in Japan. Not all schools can/do sponsor visas for overseas students, though, so this is something you'll need to check with the specific school when applying.

Student visa applications do usually require that you show you have a certain amount of financial resources (generally money a bank account) to support yourself during the time you'll be studying in Japan. While student visas holders are allowed to work while in Japan, the amount of ours they're allowed to work are limited, and the part-time jobs they can find usually aren't very lucrative, so the financial resources part of the visa requirement is to prevent people from moving to Japan as students, running out of money, and not bein able to afford a place to live/food to eat. For something like this, your grandfather could fill the role of pledging that he will financially support you. It's pretty common for student visa applicants to fulfill the financial requirement of the student visa application not through their personal finances, but with the pledged help of parents or guardians.

�uAlso, I�fm currently living in a country that is not my �ghome country�h or country of nationality. I�fm American but I�fm living and I�fve gone to high school in Mexico. Could this also be a potential problem in the application process, for the school and/or the visa?�v

This probably won't be a deal breaker, but it might slow the application process down. A visa is an agreement between two countries to let one country's citizen live in another country. If you're a U.S. national living in Mexico, your paperwork is probably going to need to go through the U.S. consulate, which might add an extra step to the process.