Is Asta speaking a specific dialect? In Black Clover, at least in the anime, Asta frequently shortens します (shimasu) into しゃす (shasu). Is that supposed to be indicative of an accent or dialect, or is there some other reason for him to do that?
In short, he is speaking colloquially in slang, not ant dialect in particular.

For an example: ナシャス。 

オナシャス (onashyasu) means お願いします (onegaishimasu, lit. please)。 
「お願いします (onegaishimasu)」

→「おねがいしゃす (onegaishyasu)」

→「おねいしゃす (oneishyasu)」

→「オネシャス (ONESHIYASU)」

→「オナシャス (ONASHIYASU)」

This is like shortening "please" to "pls" or "plx".

This speech pattern is suppose to outline how crudely (or uncouthly) Asta speaks, in contrast to the rest of the other Knights.