I mentioned a few days ago the similarities between Spanish and other Latin based languages. French is not as close to Spanish as Italian or Portuguese. but is is the language I learnt at school (a long time ago) so I have often found it useful in guessing Spanish words. Here are a few examples that have helped me.
From the French word 'plein' (full) I guessed 'pleno' at a petrol station and was understood, even though 'lleno' is probably more common. 'Pleno' would be used more often in a phrase like 'full speed'.
Reading the Spanish newspaper I could deduce the meaning of 'pais' (country) and 'rey' (king), from the French words 'pays' and 'roi'.
On TV, 'canal' means 'channel' in both languages.
Knowing the difference between 'conaitre' and 'savoir' (both meaning 'to know') helped understand the difference between 'conocer' and 'saber' (the equivalent words in Spanish). In both languages the former is to know a person or place ('to be aquainted with') whilst the latter is to know a fact.
Of course there are hundreds of other examples, and even more in other languages.