First, thank you very much for the detailed reply.

Being a speaker of Portuguese (Pt) and Spanish (Es) and given your input on the matter, I believe the analogy is far more accurate than we both believed.

While Pt and Es do share the same alphabet, there are quite many differences. At the surface level, Pt has ã, õ and ç while Es has ñ. Phonetically, Pt has ~38 phonemes while Spanish has ~20 (under the same alphabet). This leads Pt speakers to readily pick up Es while untrained Es speakers have a really hard time understanding Pt. Regarding word origins, Es was much more influenced by Arabic than Pt, which, in turn, was heavily influenced by African and Native American languages in Brazil.

In the analogy, Ukrainian would be Portuguese (different from Russian, but can still understand it (albeit not for the same reasons)) while Russian would be Spanish (far more known worldwide, can't understand the other)

Just a final language trivia. European Portuguese has nearly the same rhythm as Russian and shares nearly all phonemes. To someone untrained on both languages, they sound quite alike.

This does not hold for Brazilian Portuguese, though.