https://github.com/itstool/itstool/issues/22
--- itstool-2.0.5/itstool.in	2018-08-21 18:27:24.000000000 +0000
+++ itstool-2.0.5/itstool.in	2018-11-19 22:52:13.798661440 +0000
@@ -1060,9 +1060,13 @@
             if strict:
                 raise
             else:
-                sys.stderr.write('Warning: Could not merge %stranslation for msgid:\n%s\n' % (
+                try:
+                    sys.stderr.write('Warning: Could not merge %stranslation for msgid:\n%s\n' % (
                         (lang + ' ') if lang is not None else '',
                         msgstr.encode('utf-8')))
+                except UnicodeDecodeError:
+                    sys.stderr.write('Warning: Could not merge %stranslation for non-utf-8 msgid.\n' % (
+                        (lang + ' ') if lang is not None else ''))
                 self._xml_err = ''
                 return node
         def scan_node(node):