Remove duplicate headings in user guide translations

Thanks for fixing #765 (closed), @erciccione! I'm glad the user guide titles are now used as headings. I promised I'd clean up any duplicate headings where they were inserted manually, and I'm doing this for all languages here. Along the way, I also fixed some smaller issues I noticed:

• The order of languages in the dropdown menu was random. This looks sloppy. I'm sorting them by language code. We can't sort by language name because there are different scripts. I learned that en needs to be the first to define it as the default language, and that the order is determined in _config.yml rather than xx.yml.
• The name of the Arabic language was written in Arabic in some languages' YML files, but not in ar.yml and some others. All YML files are now using native names for all languages.
• Missing i18n key: merchants.intro3 was a recurring warning. No problem, just a new string. I'm translating this one for Dutch.
• In nl: changing flag to translated="true" for view_only.md.
• In fr: titles should consistently use lower case for normal nouns. Changing Conserver, Débutant, Portefeuille, Démon to lower case.
• In pt-br: there can't be a good reason for translating the Recovery category on the user guide overview page as if it says Backup & Recovery. Changing Backup & Recuperação to Recuperação.
• In zh-tw: changing the OS name windows to Windows.
• In ru: the multisig guide was translated (that's impressive btw), but the title was in English in ru.yml. I'm copying the Russian title from the article file while removing it there.
• There were no duplicate headings in the developer guides.

I tested all changes by building the site locally using jekyll.