From a native speaker (not me) : this sentence is completely unnatural. People say "być w stałym związku".
I thought so too, but http://www.wsjp.pl/index.phpid_hasla=4726id_znaczenia=4973581l=28ind=0
głęboki, poważny, prawdziwy; silny, stały, szczęśliwy, trwały, udany; wolny; nowy; podobny związek; związek małżeński, partnerski; oparty na miłości, szacunku
stały związek - long term relationship, https://www.diki.pl/slownik-angielskiego?q=stały+związek
Dear native English speakers is strong relationship, long-term relationship, steady relationship the same thing?
Because in Polish silny związek and stały związek are not the same.
A Strong relationship and a long term relationship would not mean the same thing. It would also be very uncommon for someone to say that are in a strong relationship in English. (atleast where I'm from)
If it was a boyfriend & girlfriend relationship they would probably say that the relationship is going well or that they are happy together instead of saying they are in a strong relationship.
A long term relationship would just mean they have been together for a long time and not necessarily implying a good relationship.
I don't know why you guys are in here, but I'm very grateful for your presence :D thanks for bringing this up