My (33F) BFF (32F) ditched me on a vacation we spent years planning, after her BF (30M) proposed on the first night of our vacation

My BFF lets call her Ann, she has a boyfriend lets call Mark. Originally we were supposed to take this trip in Early 2020 but that kind of didn't happen due to COVID. We luckily managed to get a refund on majority of things, this trip served two main purposes. Firstly for a vacation we have both wanted to do since we were young, and secondly she was going to go see a big chunk of her moms side of the family she has not seen in over a decade. It was kind of split 50/50, since this was supposed to be almost a 2 month trip I personally have been saving for years now. One thing to note is that Ann is fluent in the primary language of where we are vacationing while i am not. I can talk to some city people enough to get by, but once i leave and the slang and dialects start popping in i am lost.

It was just supposed to be me and Ann, however a last minute addition was made with her including her current boyfriend of 3 years with. This was more or less dropped on me pretty much 2 weeks before we left, I was really unhappy with this because well it messed up a lot of plans. But i begrudgingly accepted him after we reworked some stuff. However i told Ann multiple times and her boyfriend that this trip is about us not Ann and her BF.

The first night we were there Mark dropped to one knee pulled out the ring and asked her to marry him. Since then i feel like a third wheel who is no longer welcome on this trip. This has changed EVERYTHING, the trip is now basically about Ann and her new fiancée, multiple times we had a lot of plans. Including some stuff that was paid for, and instead she ended up ditching me to go spend time with Mark.

Tomorrow we are supposed to catch a train and travel for the next leg of our trip, and she wants me to go alone. Instead she confronted me and that she needs at least a week to figure out feelings and organize her thoughts so instead they booked without telling me some fancy couples thing. Instead she wants me to go on a trip alone as a woman in a country I can barely speak the language. Even worse i am basically going to have to show up to some extended part of her family alone and ask to stay there. She said its fine, but it is really not comfortable at all.

I tried to talk to her and she got really upset and told me this vacation isn't just about me, and that hurt me a lot. Now pretty much i am going to spending Christmas alone in a country where i don't speak the language. I got pretty emotional and asked her WTF am i expected to do. She got really defensive and said its just a week and we are here until early February, she will meet up with me on news year eve or the day after.

My question is how can I talk to her and make her understand that she cannot just ditch me without coming off like a massive asshole?

TL;DR; BFF ditched me on our vacation, told me to go alone now. How do I explain she cannot just do that?