JudasIscariot: Nice terms of endearment in PL:
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koteczek, kotek - kitten
котёнок, котёночек. Yep, we use it too.
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słoneczko - sunshine
солнышко. I wouldn't recommend using "słoneczko" when addressing a Russian girl unless she knows Polish well though. You see, there's also a word "слон" - "elephant" so some could misinterpret it as "слонышко", a hint to the girl having a... weight problem.
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kwiatuszek - little flower
цветочек. Possible, though I can't say I heard it used that often.
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misio/misiu - teddy bear
медвежонок. Literally "little bear/bear cub", but yes, also used for "teddy bear" ("плюшевый медвежонок") and can be endearing.
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mój skarb - my treasure
моё сокровище. We have it as well. Скарб - more like "possessions", not advisable.
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moje szczęście - my happiness
моё счастье. And this one's quite good.
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najdroższa/najdroższy - dearest
дорогая (dear). Дорожайшая is closer phonetically, but not semantically as it is somewhat archaic and formal, I believe.
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najcudowniejsza/y - most wonderful
[наи]чудеснейшая. Well... maybe. Though "чудесная" - "wonderful" is more common.
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najpiękniejsza/y - most beautiful
[наи]прекраснейшая. Probably same as the one above. Then there's "красивая" - "beautiful".
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żabko" - litte frog (no I am not kidding about this one!). Just make sure you use the "k" or you'll have "żabo" and then you are simply calling her a "frog" at this point and I am sure you know how well THAT one will be perceived.
жабка. Nah, I don't think it would be a good idea to call a girl that. Жаба means "toad", "лягушка" is "frog", both have... doubtful value as compliments.