Upgrading Nokia Lumia 1020 to Windows 10 & Project Astoria questions

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I just looked in the Microsoft store and there are still some WP8.1 apps to be found. What appears there can also be downloaded via a small detour (as an XAP file) and installed on the phone.
I hope I don't have a mistake now.

Thank you, that sounds nice.
Sorry if I'm asking stupid questions, but I'm not too much familiar with Windows platform on mobile phones, it's my father's phone (I use the phone with Android OS). Is there an easy way to install XAP files manually on the phone with no SD card (Lumia 1020).
Btw, and one important question (I forgot to ask earlier), will there be an option to also install a different language pack after the upgrade to Windows 10, and not only english?