Add doc

backend/src/translation.rs

@@ -207,8 +207,11 @@ impl Tr {
         format!("{}-{}", self.lang.code, self.lang.territory)
     }
 
+    // Return the first day of the week for the current language.
+    // See https://en.wikipedia.org/wiki/Week#First_day_of_the_week.
     pub fn first_day_of_week(&self) -> Weekday {
         match (self.lang.code.as_ref(), self.lang.territory.as_ref()) {
+            // TODO: Add more languages: https://en.wikipedia.org/wiki/Week.
             ("en", "US") => Weekday::Sun,
             _ => Weekday::Mon,
         }