feat(gui): let the UI language be chosen manually (#179)

The interface followed the system locale with no way to override it. Settings now
carry a language picker, stored as forced_lang in gui.json, with "System" listed
first so the choice stays undoable.

rust-i18n keeps the current locale in a process-wide atomic and t! reads it on
every lookup, so switching takes effect on the next frame without a restart.

Languages are listed by their own name rather than a translated one, since that
is what a reader looking for their language recognises. A test asserts every
shipped locale has such a name, so adding a translation without naming it fails
rather than silently showing a bare language code.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

pwsp-gui/src/gui/views/settings.rs

@@ -1,4 +1,4 @@
-use crate::gui::SoundpadGui;
+use crate::{gui::SoundpadGui, locale};
 use egui::{Align, Button, Color32, ComboBox, Layout, RichText, Slider, Ui};
 use egui_material_icons::icons::ICON_ARROW_BACK;
 use pwsp_lib::types::config::PreferredTheme;
@@ -64,6 +64,7 @@ impl SoundpadGui {
             // ---------- Selectors -----------
             self.draw_mic_selection(ui);
             self.draw_output_selection(ui);
+            self.draw_language_selection(ui);
 
             let mut selected_theme = self.config.preferred_theme.clone();
             ComboBox::from_label(t!("gui.settings.theme.label"))
@@ -184,4 +185,32 @@ impl SoundpadGui {
             self.set_output(selected_output);
         }
     }
+
+    fn draw_language_selection(&mut self, ui: &mut Ui) {
+        let mut selected = self.config.forced_lang.clone();
+        let previous = selected.clone();
+
+        let selected_text = match &selected {
+            Some(code) => locale::display_name(code).to_string(),
+            None => t!("gui.settings.language.system").to_string(),
+        };
+
+        ComboBox::from_label(t!("gui.settings.language.label"))
+            .height(30.0)
+            .selected_text(selected_text)
+            .show_ui(ui, |ui| {
+                // Listed first so picking a language stays undoable.
+                ui.selectable_value(&mut selected, None, t!("gui.settings.language.system"));
+                for code in locale::available() {
+                    let name = locale::display_name(&code);
+                    ui.selectable_value(&mut selected, Some(code.clone()), name);
+                }
+            });
+
+        if selected != previous {
+            self.config.forced_lang = selected;
+            self.config.save_to_file().ok();
+            locale::apply(&self.config);
+        }
+    }
 }

pwsp-gui/src/locale.rs

@@ -0,0 +1,58 @@
+use pwsp_lib::types::config::GuiConfig;
+
+/// Applies the UI language: the one pinned in the config, or the system's when none is.
+///
+/// Safe to call at any time — `t!` reads the locale on every lookup, so the next frame
+/// already renders in the new language.
+pub fn apply(config: &GuiConfig) {
+    let locale = config.forced_lang.clone().unwrap_or_else(system_locale);
+    rust_i18n::set_locale(&locale);
+}
+
+fn system_locale() -> String {
+    sys_locale::get_locale().unwrap_or_else(|| String::from("en-US"))
+}
+
+/// Locales shipped under `locales/`, in a stable order for the menu.
+pub fn available() -> Vec<String> {
+    let mut locales: Vec<String> = rust_i18n::available_locales!()
+        .into_iter()
+        .map(|locale| locale.to_string())
+        .collect();
+    locales.sort_unstable();
+    locales
+}
+
+/// How a language names itself, which is how it should appear in a language picker
+/// regardless of the language the rest of the UI is in. An unnamed locale falls back to
+/// its code, so a newly shipped translation is still selectable.
+pub fn display_name(code: &str) -> &str {
+    match code {
+        "en" => "English",
+        "ru" => "Русский",
+        "es" => "Español",
+        "fr" => "Français",
+        "zh" => "中文",
+        "ar" => "العربية",
+        "kz" => "Қазақша",
+        "he" => "עברית",
+        "pt-BR" => "Português (Brasil)",
+        other => other,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn every_shipped_locale_has_a_native_name() {
+        for code in available() {
+            assert_ne!(
+                display_name(&code),
+                code,
+                "locale {code} falls back to its code in the picker"
+            );
+        }
+    }
+}

pwsp-gui/src/main.rs

@@ -1,7 +1,8 @@
 mod gui;
+mod locale;
 
 use anyhow::{Context, Result, anyhow};
-use pwsp_lib::utils::gui::ensure_pwsp_audio_dir;
+use pwsp_lib::utils::gui::{ensure_pwsp_audio_dir, get_gui_config};
 use rust_i18n::i18n;
 use std::{
     env,
@@ -13,8 +14,7 @@ i18n!("locales", fallback = "en");
 
 #[tokio::main]
 async fn main() -> Result<()> {
-    let locale = sys_locale::get_locale().unwrap_or(String::from("en-US"));
-    rust_i18n::set_locale(&locale);
+    locale::apply(&get_gui_config());
 
     let args = env::args().skip(1).collect::<Vec<String>>();