Octane is Mexican, we all agree on that. So why does he speaks in a Madrilenian accent in the Spanish dub? (an accent from Madrid, Spain). Him speaking in that accent takes a lot from his personality.
The same applies with Wattson (she looses her French accent), Bloodhound (they loose their Northern-Germanic accent), Crypto (he looses his Asian accent), Lifeline (Jamaican-ish-like accent lost), Gibraltar (Maori accent lost) and so on. They all speak in the same Madrilenian accent, so they have lost a very important part of their personality.
I play a lot of Bloodhound and I first started picking them purely because of their accent.
Why bother making a group of legends so widely different from each other and then have them talk in the same accent, and make them loose their personality (Because Gibby is friendly not because he has friendly lines, but because his accent sells it like a good-time Hakunna-Matata dude)
To put it in perspective, imagine if these 12 characters all speak in a very RP English. They all speak perfect English with no flaws in their speech or some mention of their nationality. The only one that would benefit for having that accent is Caustic. Octane would be a Mexican dude that speaks RP English, and that would feel odd.
So why not having a Mexican Voice actor play Octane in the Spanish dub? Why not making Wattson speak Spanish with a French accent? Or Bloodhound? Or Crypto? Or anyone else?
