I think you can't properly study one without dipping your toes legs in the other, but focusing on linguistics first as a monolingual seems needlessly confusing. However, Wikipedia is surprisingly good for basic linguistics.
IMO the best way to learn a language is like a baby, just listen, watch a shit ton of TV. You can set up VLC to show subtitles in multiple languages at once. You don't even have to try to understand, you just have to listen for recurring things and eventually it will start making sense. If you want to tryhard, start out by reading the "Language X phonology" wikipedia article to try and prepare yourself for potentially confusing sounds. A lot of language education is centered on written language which is strange since language far predates writing.
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