Tusky is one of the oldest Fediverse apps (clients) we have in Android, in my opinion is the best, the one that I use.
If you publish using more than one language you can select it when writting a post in the top right corner. Default value is set in your fediverse acount preferences. If you are replying to somebody else's post then it gets original post's language setting.
However, you may want to use diferent languages to write your own posts. Then using this feature you will help automatic translation service work better in your instance (when available) and you'll also help your followers allowing them filter your posts, because they might be interested in just one of them.
This drop down list is really big, sorting names in alphabetical order. It would be much convenient if you have your most used language at the top as first choices.
I've found the solution in this Tusky issue, connieduck explains how this list works.
go to Prefered Languages and add the languages you want. The first in the list is your device's language, so if you change the order then every app in your device will also change its language. From second are the languages your device will use IF the first one is not available. So order is relevant.
no need to restart your device, back in Tusky you can see that selected languages in first step are on top. Handy.
you can still select any other language you want/need.
If you enjoy using Tusky, you can contribute its development in Opencollective
web de ghose (xmgz)