Add hacking guide

HACKING.md

+# Hacking guide
+
+## Testing different locales
+
+To test the app with a different locale, add this e.g. in `Main.kt`:
+
+```
+Locale.setDefault(Locale("ar"))
+```
+
+and replace `ar` with a different language you would like to test, such as
+Arabic in this example.
+
+It is also possible to run from the command line using Gradle with a
+different language setting:
+
+```
+GRADLE_OPTS="-Duser.language=fr -Duser.country=FR" ./gradlew run
+```
+
+## Threading
+
+Some rules about threading in Briar Desktop:
+
+* Always use local variables instead of the composable State objects inside
+  another thread: [Source](https://code.briarproject.org/briar/briar-desktop/-/merge_requests/55#note_57632)

README.md

@@ -89,6 +89,11 @@ If you want to use the JDKs downloaded by Intellij IDEA, you can for example spe
 
 See [TRANSLATION.md](./TRANSLATION.md) for more information.
 
+## Hacking
+
+See [HACKING.md](./HACKING.md) for useful information when trying to work
+with the source code.
+
 ## Maintenance
 
 From time to time, translations and Flatpak's dependencies should be

TRANSLATION.md

@@ -32,3 +32,6 @@ flatpak-builder:
 ```
 --env=LC_ALL=de_DE.utf8
 ```
+
+There's also a section in [HACKING.md](./HACKING.md#testing-different-locales)
+dedicated to testing of different locales in order to try out translations.