Are you renting right now, or did you purchase this home with your partner? I’m asking because living with her is making you miserable, and seems to also be costing you your relationship with your son. Your partner has an enmeshed, unhealthy relationship with her daughters. I wouldn’t call it ‘strong’, I’d call it detrimental to their (and your) wellbeing. You’re stuck for the moment, unless you’ve got somewhere else you can stay, but once the market has re-opened & you’re able to let a new flat I’d move out and continue to date your partner but not live together if you want to salvage the relationship. Not to mention your sanity. Your living situation sounds horrible. 

There is a lot going on here that's wrong but i will address the language issue. When i was hanging out with my SO and his family, who moved from another country to where i live about 45 years ago, we were hosting a dinner at my SO's house. English is the language spoken at home by him and his kids. All the adults in the group, including me, were speaking in English and i was part of the conversation. Things were going well. Suddenly they switched to their native language for about 5 minutes straight. I was confused. I was part of the group and suddenly i wasn't. I just sat there staring at the wall while they talked and laughed, and eventually i just started clearing plates and washing dishes just to have something to do besides stare blankly. I felt suddenly like an outsider in my SO's home at a dinner we were hosting that i cooked. Afterwards, i asked my SO to "catch me up", since i missed a lot of the conversation. He said "oh, we were talking about lawn chairs." 
 

I explained how it made me feel and against my wishes, he told the family. They supposedly had no idea i would care. But, the member who switched first was very enmeshed with BM. My preference would have been in that situation, if he would just keep me in the loop, maybe explaining to me what the joke is if everyone laughs, etc, without trying to influence what his brothers/sisters/parents did. Now i felt like not just an outsider but a whiny one. 
 

So, yes, i believe that is rude. Some people will say you are rude for expecting people to speak English because if you are uncomfortable with other languages you must be some kind of bigot. That's the reactions i've heard. But, i've been there and that is how it felt to me. 
 

This is something your SO must address. It sounds like one of the least of your problems though. The 26-year-old should not have the opportunity to do this daily at your house because she needs to launch. And they all should respect you more. Maybe they treated your son in a disrespectful way, too, and that's why he doesn't come around as much? 

‘Some people will say you are rude for expecting people to speak English because if you are uncomfortable with other languages you must be some kind of bigot.’

One could argue that it’s rude to speak in a language others can’t understand while at a gathering because it’s exclusive and alienating, which is beyond rude. Anyone who calls someone a bigot because they felt uncomfortable when excluded in that way is gaslighting. I’m sorry you have to deal with that Rumple.

I speak another language and when my family switches to the language my husband doesn't understand. I'm part of the conversation and interpreting for him at the same time.   My parent's English is very basic but they try. They can always practice when you aren't there. If she is speaking English until you walk into the room it's rude. 

If the 26yr old is making a salary and not paying rent she can help her mom afford the rental. 

Your SO should at least hear out your concerns if she truly cares about you and the relationship. If you try and she deflects at least you made an attempt to salvage the relationship. 

To me, the speaking Polish thing is such a big indicator of disrespect that I don't know how you could solve anything else until that is resolved. When the 26 year old (also, why is she still living with her mother at 26!?) switches to Polish, do your partner and her other daughter continue to converse in english? Does anyone translate for you to keep you in the conversation?

I only speak English and Spanish, but have so many friends who are multi-lingual. When we're in groups with mixed native languages, everyone makes an active effort to speak english or at least translate for anyone who didn't understand something that was said. It is considered very rude to intentionally speak a language that isn't understood by everyone present when there is a common language to everyone. I just don't understand how your partner thinks that's an acceptable behavior from her daughter? I wouldn't tolerate it.

This woman's behavior sounds nothing like the "best friend" that you seem to cherish so. 

This relationship and home life is straight up emotionally abusive. 

Definitely find your own place and consider therapy to understand why you have allowed yourself to be treated so shabby. 

Do NOT fall for sob stories on how she can't afford a place on her own. She did before you moved into this nightmare. 

Growing up in a traditional Polish home, in a Polish neighborhood where everyone spoke Plish.  What I can tell you, is that the mother is typically the matriarch.  So, the children do not listen to thier fathers. Typically the fathers worked so the mother was home dealing with the children, even when the mother worked, the children looked to her for where the boundaries lie and guidance. It wasnt wait til your father got home, it was wait until I tell your mother.  That being said I am surprised she allows her 26 year old daughter to live at home. Maybe she needs counseling as she must harbor guilt from her failed marriage.  That is not typical.  My mother and grandmothers ruled with an iron fist. My father and grandfathers were the kind fun ones. My mother has even told me although she was never punished she was pushed hard to be independent that is why she married at 18. I was on my own by 17. It has always been my experience Polish woman are very independent and strong willed. I still carry those values, they are so ingrained. One of my friends told me a story once, when her son got to be bigger than her, she had him bring her a step stool so she could be eye level with him to slap him across the face.