The app developers haven't implemented translation yet.
The official Mastodon roadmap page shows translate for Android as "in progress", which usually means it is coming fairly soon, possibly in the next update:
https://joinmastodon.org/roadmap
I don't know what the timetable is on Tusky, but according to the relevant issue (at https://github.com/tuskyapp/Tusky/issues/134) it hasn't been done yet.
One workaround might be using Fedilab for Android, which has translation?