My English and I are from the UK; US English often works differently.
…in a relationship with… Duo's short sentence "Adam jest w związku z Martą" is most closely translated by
- "Adam is in a relationship with Marta", which clearly indicates that
- "Adam and Marta are a couple", as the broader
- "Adam and Marta have a relationship" might or might not imply – this could instead be a business or working relationship, perhaps.
- "Something's going on between Adam and Marta" might imply all of the above, or even that they're in the same orchestra or criminal gang…
It's safest to learn "A is in a relationship with B" as a set English expression. Generally I encourage language learners to use clear expressions that avoid ambiguity and embarrassment, but if you want to muddy your meaning, leave out a and try
…in relationship with… – which might mean relative to; with respect to.
- "Adam is in relationship with Marta" doesn't work in (UK) English, though
- "Adam is in partnership with Marta" (i.e. business relationship) is fine.
It's difficult to find good constructions that work without a, suggesting that it's very much minority usage, but try…
- "Adam was in relationship with 8 other women before he finally met Marta."
(here I prefer …in relationships with… to hint that many of the relationships may have been at different times)
- "People tend to behave differently in isolation or in relationship with other people."
- "In a car engine, any single part has to function correctly in relationship with thousands of others."
- "Company A is in relationship with Company B." [?US usage]