When you press X, it INSTANTLY skips to the end of the current textbox. Same with holding C, it doesn't go through each of the letters but instead progresses past through each textbox quickly. So, essentially, language setting doesn't make a difference. It MIGHT make a difference on the "Susie joined the team!" textbox and the textbox after that one ("Susie followed at a distance as to not be associated!") but it probably doesn't due to the fanfare.

It actually does. Check this thread https://www.speedrun.com/deltarune/thread/9utuj

oh it does

At the start (before Toriel speaks) there is less text in every text box in the Japanese version so you can get through text boxes quicker than the English ones.

1. Language does matter.
2. If you can't skip text boxes, mashing doesn't matter.

Japanese saves like 13-15 seconds, not too much but still