MOBILE-4362 scripts: Fix lang index keys

Co-authored-by: Pau Ferrer Ocaña <crazyserver@gmail.com>

scripts/update_lang_functions.sh

@@ -22,7 +22,7 @@ function copy_lang {
     lang=$1
 
     index_keys=$(jq -r 'to_entries[] | "\"\(.key)\","' langindex.json)
-    index_keys=$(echo "$index_keys" | sed 's/,*$//')
+    index_keys=${index_keys:0:-1}
 
     hyphenlang=${lang/_/-}
     langfilepath=$LANG_PATH/$hyphenlang.json