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 — Settings → System → Permissions and access. Tap it to allow access; if you refused before, it opens the game's page in your phone settings.
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 Permissions and 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.
How to talk in voice chat and with your clan — see How does voice chat work? How do I talk to my clan?