Add client setting for whether to sync locale settings with server in multiplayer

src/lang/english.txt

@@ -2370,6 +2370,9 @@ STR_CONFIG_SETTING_LOCALISATION_UNITS_HEIGHT_IMPERIAL           :Imperial (ft)
 STR_CONFIG_SETTING_LOCALISATION_UNITS_HEIGHT_METRIC             :Metric (m)
 STR_CONFIG_SETTING_LOCALISATION_UNITS_HEIGHT_SI                 :SI (m)
 
+STR_CONFIG_SETTING_SYNC_LOCALE_SETTINGS_NETWORK_SERVER          :Sync localisation settings with server in multiplayer: {STRING2}
+STR_CONFIG_SETTING_SYNC_LOCALE_SETTINGS_NETWORK_SERVER_HELPTEXT :When joining a multiplayer game as a network client, change the localisation settings to match the server
+
 STR_CONFIG_SETTING_LOCALISATION                                 :{ORANGE}Localisation
 STR_CONFIG_SETTING_GRAPHICS                                     :{ORANGE}Graphics
 STR_CONFIG_SETTING_SOUND                                        :{ORANGE}Sound

src/saveload/afterload.cpp

@@ -4069,6 +4069,10 @@ bool AfterLoadGame()
 
 	if (_networking && !_network_server) {
 		SlProcessVENC();
+
+		if (!_settings_client.client_locale.sync_locale_network_server) {
+			_settings_game.locale = _settings_newgame.locale;
+		}
 	}
 
 	/* Show this message last to avoid covering up an error message if we bail out part way */

src/settings_gui.cpp

@@ -1750,6 +1750,7 @@ static SettingsContainer &GetSettingsTree()
 			localisation->Add(new SettingEntry("locale.units_force"));
 			localisation->Add(new SettingEntry("locale.units_height"));
 			localisation->Add(new SettingEntry("gui.date_format_in_default_names"));
+			localisation->Add(new SettingEntry("client_locale.sync_locale_network_server"));
 		}
 
 		SettingsPage *graphics = main->Add(new SettingsPage(STR_CONFIG_SETTING_GRAPHICS));

src/settings_type.h

@@ -762,6 +762,11 @@ struct ScenarioSettings {
 	bool house_ignore_grf;                   ///< allow manually adding houses regardless of GRF restrictions
 };
 
+/** Settings related to currency/unit systems. */
+struct ClientLocaleSettings {
+	bool sync_locale_network_server;         ///< sync locale settings with network server
+};
+
 /** All settings together for the game. */
 struct GameSettings {
 	DifficultySettings   difficulty;         ///< settings related to the difficulty
@@ -785,6 +790,7 @@ struct GameSettings {
 /** All settings that are only important for the local client. */
 struct ClientSettings {
 	GUISettings          gui;                ///< settings related to the GUI
+	ClientLocaleSettings client_locale;      ///< settings related to used currency/unit system in the client
 	NetworkSettings      network;            ///< settings related to the network
 	CompanySettings      company;            ///< default values for per-company settings
 	SoundSettings        sound;              ///< sound effect settings

src/table/settings/settings.ini

@@ -4101,6 +4101,14 @@ def      = nullptr
 post_cb  = [](auto) { MarkWholeScreenDirty(); }
 cat      = SC_BASIC
 
+[SDTC_BOOL]
+var      = client_locale.sync_locale_network_server
+flags    = SF_NOT_IN_SAVE | SF_NO_NETWORK_SYNC
+def      = false
+str      = STR_CONFIG_SETTING_SYNC_LOCALE_SETTINGS_NETWORK_SERVER
+strhelp  = STR_CONFIG_SETTING_SYNC_LOCALE_SETTINGS_NETWORK_SERVER_HELPTEXT
+cat      = SC_BASIC
+
 [SDT_BOOL]
 var      = vehicle.adjacent_crossings
 def      = true