How do I change my nickname, language, region and microphone settings?
Language — Settings → Basic → Change Language.
Server region — Settings → Basic → Change Region.
Nickname — Settings → System → Change Nickname.
Headset Mic (capture your voice through a Bluetooth headset instead of the device's own microphone) — Settings → Audio → Headset Mic. Off by default.
Microphone access (the system permission the game needs) — Settings → System → Microphone Access.
All of these except Headset Mic are available from the lobby only. Language and Region are in the Basic tab's Main section, which isn't shown during a match, and Nickname and Microphone Access are on the System tab, which the in-match settings screen doesn't include.
If teammates can't hear you, start with Microphone Access — without that permission the game has no microphone at all. If you're wearing a Bluetooth headset and still aren't heard, turn Headset Mic on.
What's shown on screen in game is on the Basic tab under In Game Display, and it is available during a match: Compass, FPS Counter, Displaying ID and Server Name in the game, Show Players ID, Interaction Markers, Off-Screen Marker.