This topic comes up often enough in /3, so here is the skinny.
There are two way to "talk" to the enemy.
1. Use a Private Store. Words on your sign are not translated.
2. The filter that changes the enemies speech into gobbledygook does not alter the first letter of each word, so if you s p a c e o u t what you want to say, the filter does not alter it.
There is a third, of course... vent. That, however, is cross-realming and, if they ever find out about it, it can get you or even your entire Legion banned.