German.

Spanish seems pretty easy to learn honestly. I think English is extremely similar to Spanish. I never sat down to learn any sentence structure in Spanish for example. It's just English in slightly freer. Half the words in Spanish are also in English, so a lot of the vocabulary is easy to remember.

So I would say I learned a lot of Spanish "through English".

Some things are more similar between German and Spanish though.

Obviously German has genders like Spanish and they work in the same way. But in German you can never tell from the ending of the word like you can in Spanish. So every word in German is like coche / noche, where you just have to know the gender by heart. Spanish seems quite simple here, too.

A few grammatical constructs are nearly identical in Spanish and German. Reflexive verbs/pronouns work the same way. me gusta = mir gefällt, te gusta = dir gefällt, me gustas = mir gefällst (du) etc.
I also found that the reflexive pronouns themselves are eerily similar. me = mich, te = dich, se = sich. Sameish starting letters at least. Anyway, it made it very easy to remember it.

Very rarely there is a word that is the same in German and Spanish, but not in English, like Tinte/tinta or egal/igual (obviously German has both from Latin/French).

Aside from that I also found that the media is more similar between German and Spanish as opposed to English because English media usually tends to be more conservative and tabloidy.