This entire verse is written as third-party commands and is a good example of why "must" works well in translating such statements instead of "let them". This verse starts the same as Matthew 24:16, but then add more detail not seen in the other versions of this verse. The KJV is more accurate than many more recent translations. Many of them add the idea of fleeing the city here when the verse is clearly about fleeing the nation of Judea. The verse also has a unique word, as so much of Luke does.

The word translated as "then" means "at that time" when referencing a specific time or "then" in the continuation of a story.

The word translated as "them which are" is from the Greek article, "the," which usually precedes a noun and, without a noun, takes the meaning of "the one" or, in the plural, "the ones."

The word translated as "in" also means "within", "with," or "among."

"Judaea" is translated from a Greek word that means "a Jew", "Jewish", "Judea," or "Jewish." Judea represents civilization.

"Flee" is translated from a Greek word that means "to flee", "escape," and "to take flight." It is in the form of a third person command, which isn't used in English. We would typically use the second person command instead or say that someone "must" do these things.

The word translated as "into" means "into" a place, "towards" as a direction, and "up to" limits in time and measure.

The word translated as "mountain" means "mountain" or "hill" but for those raised in Egypt, as Christ was, it also means "desert." In the forms that Christ uses when talking about "moving mountains," it could be a homonym that means "mule" but the form here, with the article, could only mean "mountains."

The Greek word translated as "and" is used as the conjunction "and", but it also is used to add emphasis ("also"). In a series, it is best translated as "not only...but also." After words implying sameness "as".

The word translated as "them which are" is from the Greek article, "the," which usually precedes a noun and, without a noun, takes the meaning of "the one" or, in the plural, "the ones."

The word translated as "in" also means "within", "with," or "among."

The word translated as "the midst" generally means "middle" but has a lot of special meanings with different prepositions. One of those with the "in" is "offer for competition" and "middle point." It also means "difference."

The word translated as "it" is the Greek word commonly translated as third-person pronouns in English. It clearly refers to the country here, but in other Bibles is taken to mean the city.

"Depart out" is  a unique word for Jesus which means  "depart", "slip out", "give way", and "cede". The literal sense of "slip out" seems light-hearted given the context. This is probably why Jesus uses it. It is in the form of a third person command, which isn't used in English. We would typically use the second person command instead or say that someone "must" do these things.

The Greek word translated as "and" is used as the conjunction "and", but it also is used to add emphasis ("also"). In a series, it is best translated as "not only...but also." After words implying sameness "as".

The word translated as "them which are" is from the Greek article, "the," which usually precedes a noun and, without a noun, takes the meaning of "the one" or, in the plural, "the ones."

The word translated as "in" also means "within", "with," or "among."

The word translated as "countries" is uncommon. The word means "a place", "a spot" and "a landed estate".  This word is not used in Mark or Matthew, but it is used in John and  Luke 15:13 where it is translated as "country".  However, the basic idea of "places" works here, referring to the mountains and where those in the middle of the country slip out to.

The negative "not" used here is the Greek negative of a subjective opinion, commands, and requests. The sense is that "you don't want" to do something, not that it isn't done or don't think something that might be true. If it wasn't done or wasn't true, the objective negative of fact would be used.

"Enter" is a word that means "go or come into" and has the double meaning of "coming into one's mind." It is in the form of a third person command, which isn't used in English. We would typically use the second person command instead or say that someone "must" do these things.

"Thereinto" is two Greek words that mean "into it." The word translated as "unto" means "into" a place, "towards" as a direction, "in regards to" a subject, and "up to" limits in time and measure. The word translated as "it" is the Greek word commonly translated as third-person pronouns in English.  The word means "the same" when used as an adjective. In the adverbial form, it  means "just here" or "exactly there."