This sure seems like a bug but I thought it would be reported by now.

I have a test board with phpBB 3.0.6. I install the French language pack. In the User Control Panel, I change my language from English to French. Text changes work fine. However, buttons like the "New Topic" button where the word is embedded in the image do not change. See attached screen shot. Note the New Topic button which should be in French is still in English.