patch 8.1.0510: filter test fails when $LANG is C.UTF-8

Problem:    Filter test fails when $LANG is C.UTF-8.
Solution:   Set 'helplang' to "en" for any C language. (Christian Brabandt,
            closes #3577)

src/version.c

@@ -792,6 +792,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    510,
 /**/
     509,
 /**/