Those are two different things. A cultural difference is language. A cultural strength is when some part of a culture is "strong" in the sense of being vibrant or alive or thriving.

The easiest way to determine if something is racist is to ask if you're generalizing about a race of people. If you say "all Chinese people <blank>", it is a racist statement. Most people don't like to hear that, though, because most people make racist statements without realizing it.

Is it racist to hire someone because you prefer people from their ethnic background? Yes, because again, you're making the decision based on race in general instead of the individual person. You may find out the Russian is part of the Russian mob while the Chinese person has high business ethics. You wouldn't hire just white people because you feel uncomfortable dealing with black people, so don't make the same biased decisions about Russian vs Chinese.

Probably. Would you refuse to hire a chinese programmer who speaks fluent russian (or even just really good russian)? (if you say "there's no such thing" that is racist fyi). The cultural differences mean communication difficulties is a bit of a strawman. There are lots of cultural differences your statement ignores: country folk vs city folk within russia probably have differences (they do most places), those born pre and post communism probably have cultural differences, etc.

(Warning - attempts at trying to draw me down a semantic rabbit hole about what culture is are very much distracting from this conversation and will be ignored, and probably should be ignored by others as they are pretty blatantly strawmen).

Pretending differences don't appear amongst different groups of people is ignorant. However using this to justify all groupings is also ignorant. For one thing there are different types of groupings, some valid, some not. Some reflect inherently true things, some statistically probable things, and some that are not inherently true but the result of various other factors combined with an arbitrary and otherwise insignificant grouping. For another, a lot of groupings don't lend themselves to generalizations - for instance in the US in California especially, being ethnically Chinese doesn't imply being an immigrant, or having recently immigrated family, so assuming that a person who is ethnically Chinese speaks Mandarin is a bad assumption based on a bad generalization from a grouping that probably doesn't even matter.