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Your sub-domains will be different websites altogether and when you submit them in webmasters you can mention the language & geo for each of them. I guess doing this will not impact the ranking of your brand keywords.

I would also suggest you to read this - https://mza.bundledseo.com/community/q/subdomains-or-subfolders-for-language-specific-sites

I manage an e-commerce website which asks a new user for his/her city by throwing up a pop-up. Please check yourself by visiting www.furlenco.com.

Will this pop-up have any effect on the seo?