First, a quick answer, and then more details:
They are the same sentence. One of them simply omits "of the", but it's understood.
Technically, 1 means people in general.
That is not strictly correct to say it means "people in general". Notice the first sentence includes the phrase "who I know". You didn't need to say "who I know", but you did. Therefore, (1) refers to the small subset of people in the world compromised of the people who you know. If you wanted to refer to "people in general", then it should instead be: "Most people don't know English."
Why I think 1 means people in general is because there is no THE attached to PEOPLE. In essence, it means ALL PEOPLE WHO I KNOW.
That is not strictly correct to say it means "all people who I know". You included the word "most", which is similar to "all", but certainly not the same. If there are 100 people, then "most" people are maybe 95 of them. Thus "all people who I know" would be 100 people, but "most people who I know" would be 95. So they are different.
Continuing to the other examples:
People I work with don't speak English. (all people I work with)
Not necessarily "all people". It's somewhat ambiguous if this is "all", or "most", or "some". It's open to interpretation.
The people I work with don't speak English. (some specific people among those I work with)
If we can agree "the" refers to a specific group... the group under discussion is "the people you work with". Because you don't work with 8 billion people, but rather 10 people, then "the people you work with" is the 10 people.
If you say "The people I work with don't speak English" it means "The 10 people I work with don't speak English" which is approximately the same as "all people I work with don't speak English". It's not the same as "some people among those...".
"the people you work with" is not the same as "a subset of people you work with", and is much closer to saying "all the people you work with". However, the word "all" is sort of emphatic and absolute. By removing "all" we leave room for the possibility that you meant "most".