Add note about pkgutils dependency on PyQt6 for qtlupdate

i18n/README.md

@@ -48,6 +48,7 @@ instance `en_GB` for British English.
 You will need the translation tool Qt 5 Linguist on your system.
 
 For Ubuntu/Debian, run:
+
 ```bash
 sudo apt install qttools5-dev-tools
 ```
@@ -75,6 +76,7 @@ The `i18n` folder at the root of the repository contains the `nw_XX.ts` translat
 
 Whether to add a new language to the translation framework, or to  update the file against the
 current source code, you must first run the `qtlupdate` command:
+
 ```bash
 python3 pkgutils.py qtlupdate i18n/nw_XX.ts
 ```
@@ -86,6 +88,10 @@ be a valid ISO language code, otherwise novelWriter will not accept the file.
 For instance, the French translation uses the language code `fr_FR`, so its translation file will
 be `nw_fr_FR.ts`
 
+Note: The `qtlupdate` command needs the `lupdate` tool provided by PyQt6, which uses the latest
+TS file format. The tool in PyQt5 generates an older file format. On Debian/Ubuntu it is provided
+by the package `pyqt6-dev-tools`.
+
 
 ### Edit the Translation File in Qt Linguist
 
@@ -100,6 +106,7 @@ Please select "English" and "United Kingdom" as the _source_ language if prompte
 The application does not use `.ts` files directly. The `novelwriter/assets/i18n/nw_XX.qm` files are
 the actual files used to translate the GUI into another language other than the default British
 English. These files are not generated by default, but they can be built with:
+
 ```bash
 python3 pkgutils.py qtlrelease
 ```