Learning a Foreign Language ... in a Foreign Language
This idea of learning a foreign language in a foreign language is interesting to me. I've seem some language learning materials over the years which advise that we should use only our native language. But, depending on the language, I disagree. After scratching the surface of Tagalog, I can tell that understanding Spanish is a great advantage. If I found Spanish materials teaching Tagalog, I see no reason to ignore them. The same would go for Catalan, which is so closely related to Spanish. When I run across a website written in Catalan, I already understand most of it. Why would I ignore materials I run across in Spanish that might teach me more about Catalan?
I did watch some YouTube videos in Spanish while I was learning Russian. I didn't use Spanish as the primary language to learn Russian. But, I certainly didn't find it confusing to Russian language videos in Spanish instruction. If the concept didn't make sense, I was still learning some words in both languages.
I'm not fluent enough in Russian that I can learn much about another language. But, I already know enough to recognize words in other Slavic languages so I would see no reason to ignore my skills in Russian in picking up a second language from Eurasia. Also, though I'm not at all fluent in Russian, I've already found a Russian speaker's Mandarin Chinese lessons very helpful. I think the way at least that Russian spells words makes more sense than pinyin (English).