I'm using weblate.org. It's open source so it made the most sense to me to use for an open source Android app. It can also be used for free. It's not trivial to use but can be managed.
Also various services such as Crowdin have different policies on the usage of string (usage across projects etc.).
That depends on the software:
If you tell us which app you want translate the communitiy can give you a more specific answer.
Yep, I'm working on getting the project ready for localisation now. The tricky part is separating the strings into Prism/Prologue (they're a monorepo/shared project with a bunch of shared views). They might have to be translated together, unfortunately.
I'll definitely be using some sort of crowd sourcing for translations. I know there are a fair few platforms for this out there (e.g. weblate) - you wouldn't happen to have any recommendations?
There are comercial service providers that offer a web gui
These services are free for opensource projects and cost money for comercial projects: