locale: fix missing en_US.UTF-8 locale

Summary:
See #814.
The "en_US.UTF-8" locale is not always valid. WSL uses `C.UTF-8` locale.
Change the `LANG` environment calculation to:
- Respect users' LANG setting if it's already UTF-8.
- Fallback to `C.UTF-8` instead of `en_US.UTF-8`, which works in WSL.

Reviewed By: evangrayk

Differential Revision: D52927838

fbshipit-source-id: c9110437e00382bb84e50d6b141fe5d4ce69f813

addons/isl-server/src/Repository.ts

@@ -1261,27 +1261,32 @@ function getExecParams(
   if (EXCLUDE_FROM_BLACKBOX_COMMANDS.has(commandName)) {
     args.push('--config', 'extensions.blackbox=!');
   }
+  const newEnv = {
+    ...options_?.env,
+    ...env,
+    // TODO: remove when SL_ENCODING is used everywhere
+    HGENCODING: 'UTF-8',
+    SL_ENCODING: 'UTF-8',
+    // override any custom aliases a user has defined.
+    SL_AUTOMATION: 'true',
+    SL_AUTOMATION_EXCEPT: 'phrevset', // allow looking up diff numbers even in plain mode
+    // Prevent user-specified merge tools from attempting to
+    // open interactive editors.
+    HGMERGE: ':merge3',
+    SL_MERGE: ':merge3',
+    EDITOR: undefined,
+    VISUAL: undefined,
+    HGUSER: undefined,
+    HGEDITOR: undefined,
+  } as unknown as NodeJS.ProcessEnv;
+  let langEnv = newEnv.LANG ?? process.env.LANG;
+  if (langEnv === undefined || !langEnv.toUpperCase().endsWith('UTF-8')) {
+    langEnv = 'C.UTF-8';
+  }
+  newEnv.LANG = langEnv;
   const options: execa.Options = {
     ...options_,
-    env: {
-      ...options_?.env,
-      ...env,
-      LANG: 'en_US.utf-8', // make sure to use unicode if user hasn't set LANG themselves
-      // TODO: remove when SL_ENCODING is used everywhere
-      HGENCODING: 'UTF-8',
-      SL_ENCODING: 'UTF-8',
-      // override any custom aliases a user has defined.
-      SL_AUTOMATION: 'true',
-      SL_AUTOMATION_EXCEPT: 'phrevset', // allow looking up diff numbers even in plain mode
-      // Prevent user-specified merge tools from attempting to
-      // open interactive editors.
-      HGMERGE: ':merge3',
-      SL_MERGE: ':merge3',
-      EDITOR: undefined,
-      VISUAL: undefined,
-      HGUSER: undefined,
-      HGEDITOR: undefined,
-    } as unknown as NodeJS.ProcessEnv,
+    env: newEnv,
     cwd,
   };