Translation:They were going to go to France on vacation.
Well - we don't get to know the end of the story here... They planned the trip, but the Swedish phrase doesn't say if they actually made the trip or if they had to cancel it. "They would go"/"They were going to go"/"They would be going to go"/"They were going"/"They planned a trip"/"They had been planning a trip"/"They were planning a trip"/"They were on their way"... All of these phrases (and a bunch of others) could probably be used, depending on the context.
As far as I know it ought to be accepted, because it could be a valid translation of the Swedish phrase (and indeed I would definitely translate "They were going to France on holiday" as "De skulle åka till Frankrike på semester", especially if the English phrase was followed by something that caused a change of plans). I'm not a part of the DL team though, so I can't check what the accepted versions are.
Yes. It's okay to use dom instead of de/dem, but it tends to be frowned upon in formal writing. A rule of thumb I use when figuring out if it should be de or dem is by interchanging in with vi and oss.
"De/Vi satt vid stranden" is grammatically correct. "Dem/Oss satt vid stranden" is not. The same can be seen with other phrases. "Hon var med dem och de såg inget" or "Hon var med oss och vi såg inget" are both grammatically correct. If de/dem or vi/oss were in the opposite places it would be grammatically incorrect.
If you want to use dom instead it's preferred to only use dom, generally no de and especially no dem.
Hope that makes it a little bit easier for you!
I am from Sweden and i would say if you use "de/dem" instead of "dom" when your talking about others it would be like distinguish the origins of the group you are talking about.
For example, You are helping a few immigrants with a buss ticketby talking to the buss driver , Then you should say "dom behöver bussbiljetter" (they need bustickets) to the bus diver, If you do it the way around and say "de behöver bussbiljetter" then for the buss driver it would be the same as saying "(Those people or That kind of people) need bus tickets" it works he would understand you, but he would see you as a racist, morally wrong, or unethical.
That's the same as if you speak it or type it in text,
Here are an example that could help out:
(You tell me) "Dom" tycker om öl ("They" like beer)
(I tell you) Om "dom" blir för fulla så kan "dom" spy (If "they" get to drunk "they" might puke)
(You yell so everyone hears you, while looking at me) Låt oss se vem som blir berusad först? "De" vinner ändå! (Lets see who gets drunk first? "They" will win anyway!)
(One angry parent to another!) Å nej, de spydde ner hela huset igen! (Oh no! they puked all over the house, again!)
I hope this somehow helps or shows a way on how you could think about it!
In this case i had replaced the (De) to (Dom) skulle åka till Frankrike på semester. even if a book would tell you (De)
Hmmm.... I'm not sure "åka" would be translated as "drive" though - especially not the other way around. "drive" is generally "köra", whereas "go" could be "gå", "åka", "fara" and "resa". I don't know why "travel" is considered wrong, as it's definitely the best translation of "åka", "fara" and "resa" in my opinion (but I guess most other Swedes would use "go", simply because they never use "travel" at all).