diff --git a/arcanist/linter/LocaleDependenceLinter.php b/arcanist/linter/LocaleDependenceLinter.php
--- a/arcanist/linter/LocaleDependenceLinter.php
+++ b/arcanist/linter/LocaleDependenceLinter.php
@@ -4,6 +4,38 @@
  * Check for locale dependent functions.
  * Unnecessary locale dependence can cause bugs that are very tricky to isolate
  * and fix.
+ *
+ * Be aware that bitcoind and bitcoin-qt differ in terms of localization: Qt
+ * opts in to POSIX localization by running setlocale(LC_ALL, "") on startup,
+ * whereas no such call is made in bitcoind.
+ *
+ * Qt runs setlocale(LC_ALL, "") on initialization. This installs the locale
+ * specified by the user's LC_ALL (or LC_*) environment variable as the new
+ * C locale.
+ *
+ * In contrast, bitcoind does not opt in to localization -- no call to
+ * setlocale(LC_ALL, "") is made and the environment variables LC_* are
+ * thus ignored.
+ *
+ * This results in situations where bitcoind is guaranteed to be running
+ * with the classic locale ("C") whereas the locale of bitcoin-qt will vary
+ * depending on the user's environment variables.
+ *
+ * An example: Assuming the environment variable LC_ALL=de_DE then the
+ * call std::to_string(1.23) will return "1.230000" in bitcoind but
+ * "1,230000" in bitcoin-qt.
+ *
+ * From the Qt documentation:
+ * "On Unix/Linux Qt is configured to use the system locale settings by default.
+ *  This can cause a conflict when using POSIX functions, for instance, when
+ *  converting between data types such as floats and strings, since the notation
+ *  may differ between locales. To get around this problem, call the POSIX
+ *  function setlocale(LC_NUMERIC,"C") right after initializing QApplication,
+ *  QGuiApplication or QCoreApplication to reset the locale that is used for
+ *  number formatting to "C"-locale."
+ *
+ * See https://doc.qt.io/qt-5/qcoreapplication.html#locale-settings and
+ * https://stackoverflow.com/a/34878283 for more details.
  */
 final class LocaleDependenceLinter extends ArcanistLinter {
 
diff --git a/src/qt/bitcoin.cpp b/src/qt/bitcoin.cpp
--- a/src/qt/bitcoin.cpp
+++ b/src/qt/bitcoin.cpp
@@ -219,6 +219,7 @@
     : QApplication(qt_argc, const_cast<char **>(&qt_argv)), coreThread(nullptr),
       optionsModel(nullptr), clientModel(nullptr), window(nullptr),
       pollShutdownTimer(nullptr), returnValue(0), platformStyle(nullptr) {
+    // Qt runs setlocale(LC_ALL, "") on initialization.
     RegisterMetaTypes();
     setQuitOnLastWindowClosed(false);
 }