Превод - Полски-Английски - spoko morze cieple. Bylem z bartkiem na imprezie....

hardly managed? This is not correct in English. Did the person make it to the plane or not?

I used the expression "hardly managed to catch the plane" in order to express "only just" or "almost not". So the person did manage to catch the plane, but with difficulty. "Hardly", negative in meaning, means "almost not", e. g. "There's hardly any bread left" = almost no (but still there is some left). (See: Hornby, Oxford Advanced Learner's Dictionary of current English, Oxford, 1998, p. 391.)
Quirk and Greenbaum in their Concise Grammar of Contemporaty English (New York, 1999, s. 187) point out that "hardly" is a word that "is negative in meaning" but when used together with a verb it expresses e. g. difficulty (and does not negate the verb)(e. g. "I hardly understood what she meant" = almost not.)
The authors also say the word "hardly" often functions as a DOWNTONER or MINIMISER ,Ibid., s. 218, that "has a lowering effect on the force of the verb but does not negate the verb", which is exactly the case in the sentence translated by me.
Still, perhaps for the native speaker of English, the expression doesn't sound correct? I am eager to hear your opinion.

I was wondering about another thing in this sentence; do you think the message that the person actually did manage to catch the plane would be conveyed in a better way if I changed the translation to: "I hardly managed to catch the plane that was leaving at 5" (instead of: " that was suppossed to leave at 5" ). I have a feeling that it is the very phrase "the plane that was suppossed to leave at 5" that perhaps causes the problem (i.e. because of "suppossed" we do not know if the plane left or not).
Looking forward to hearing your opinion.

"hardly" is only used in negative expressions. I'll edit the text.