Set exec LANG via rest API only

src/apps.js

@@ -2460,10 +2460,12 @@ async function createExec(app, options) {
         // When passing binary data, tty must be disabled. In addition, the stdout/stderr becomes a single
         // unified stream because of the nature of a tty (see https://github.com/docker/docker/issues/19696)
         Tty: options.tty,
-        Cmd: cmd,
-        Env: [ 'LANG=C.UTF-8' ]
+        Cmd: cmd
     };
 
+    // currently the webterminal and cli sets C.UTF-8
+    if (options.lang) createOptions.Env = [ 'LANG=' + options.lang ];
+
     return await docker.createExec(app.containerId, createOptions);
 }