Again:
1) the whole Torah is definitely NOT bein adam l’chavero. As for what Hillel responded to that Non-Jew, there are many, many mefarshim, answers. The place is not here. It can obviously not be taken at face value.
2) As I pointed, sefarim, so numerous, each list certain aspects that need to be mended. The famous one of the Ohr Hachaim – limud haTorah. Others point to Kedushah – tznius. Shmiras halashon is one. Of many.
3) It’s not the job of a Jew to be a so called “light unto the nations”. We are bidden to do the mitzvohs of HaShem, because that is His willing. Nothing more, nothing less. Incidentally, while scrupulously fullfilling HaShem’s commandments, we become a “light unto the nations”, but it’s not our focus, or goal at all.
4) Sure, we have way to go in Shmiras Halashon, as you pointed out. However, you failed to mention the so many communities where there are groups so diligently studying (and eventually practicing) these complex laws. Is there even such a concept by the non-Jews? To study laws in order to KEEP THEM?!? Refraining from derogatory speech, even where the subject is NOT PRESENT?!? Not hurting someone’s feelings?!? WHO but us YIDDEN?