Steam doesn’t advertise games that aren’t translated to those speakers. If the store page is translated but not the game, the page gets a warning saying that the game isn’t translated.

So if it wasn’t already happening, it won’t be a problem.

Pretty sure you could narrow down the individual reviews displayed by language, but the calculated rating were still all reviews or by time frame.

Some games may have translation issues, specific cultural references and so on. Valve feel this language-based review score will help to capture these issues and that "Calculating a language-specific review score means that we can better distil the sentiment of these different groups of customers, and in doing so, better serve potential customers that belong to those groups".