Document how to translate and test translations

docs/translations.md

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+# Translations
+
+Translations are stored in .ts files in the [src/translations/](../src/translations/) directory.
+
+## Adding a new language
+
+- Add a .ts file to the `TS_FILES` variable in [src/CMakeLists.txt](../src/CMakeLists.txt)
+- Run `make lupdate` in the build directory to create the .ts file
+- Translate the content of the .ts file with Linguist
+
+## Testing a translation
+
+- Build and install the app, preferably in a directory where you don't need to be root to install. This example assumes you install it to `$HOME/tmp/nanonote-inst`:
+    ```
+    mkdir build
+    cd build
+    cmake -DCMAKE_INSTALL_PREFIX=$HOME/tmp/nanonote-inst <path/to/nanonote/tree>
+    make
+    make install
+    ```
+
+- Run the app, possibly with the `LANGUAGE` variable set to force the language:
+    ```
+    LANGUAGE=<the_lang_to_test> $HOME/tmp/nanonote-inst/bin/nanonote
+    ```
+
+- If something is wrong: fix the translation, run `make install` and try again