I18N: Revert part of d95486af95 removing check for main thread.

Part of the checks to see if translating a UI string is allowed or not
is that current thread is the main one. This was a requirement years ago
of the Boost backend for translations.

Not sure whether this can be removed or not now, but it needs to be
carefully checked, done it as its own commit, and not in a beta release
branch. ;)

Sorry for the noise, totally missed this during the patch review
yesterday.

source/blender/blentranslation/intern/blt_translation.c

@@ -65,8 +65,7 @@ const char *BLT_pgettext(const char *msgctxt, const char *msgid)
 bool BLT_translate(void)
 {
 #ifdef WITH_INTERNATIONAL
-  return true;
-  /* return BLI_thread_is_main(); */
+  return BLI_thread_is_main();
 #else
   return false;
 #endif