feat(locale): add Fluent translations

Default to the platform language with persisted overrides across native and web builds.

Refs #630

src/app/update/file.rs

@@ -288,6 +288,7 @@ impl OpenCADStudio {
             pick_drag_rect: self.pick_drag_rect,
             bg_color: self.default_bg_color.map(f4_to_u3),
             paper_bg_color: self.default_paper_bg_color.map(f4_to_u3),
+            language: self.language,
         }
     }
 
@@ -317,6 +318,9 @@ impl OpenCADStudio {
         self.pick_drag_rect = s.pick_drag_rect;
         self.default_bg_color = s.bg_color.map(u3_to_f4);
         self.default_paper_bg_color = s.paper_bg_color.map(u3_to_f4);
+        if crate::i18n::set_language(s.language).is_ok() {
+            self.language = s.language;
+        }
         // Push the restored background onto every drawing tab that exists now
         // (the start tab and any initial drawing). Tabs created later pick it
         // up via `apply_bg_default` at their construction site.

src/app/update/mod.rs

@@ -4323,6 +4323,22 @@ impl OpenCADStudio {
                 Task::none()
             }
 
+            Message::LanguageChanged(language) => {
+                if self.language == language {
+                    return Task::none();
+                }
+                match crate::i18n::set_language(language) {
+                    Ok(()) => {
+                        self.language = language;
+                        self.persist_settings_if_changed();
+                    }
+                    Err(error) => self
+                        .command_line
+                        .push_error(&format!("Unable to change UI language: {error}")),
+                }
+                Task::none()
+            }
+
             Message::AboutOpen => {
                 self.active_modal = Some(super::ModalKind::About);
                 Task::none()