Aw, thanks! I'm really glad that you are enjoying the comic so far. Hopefully the next page won't be delayed for too much longer... I've been so busy with work lately, it has been difficult to find the time to draw. There need to be more hours in the day!
He's not "speaking in human." He's still speaking in Tiger language. However, now that he has been travelling with the rat for a while, she is learning to understand his language. Therefor, instead of being represented by symbols (which are meant to simulate the language gap between two people/animals that can not understand one another), his speech is now represented by using language that the reader can understand.
Also, the use of capital letters, the excessive punctuation, and the facepalm icon is rather rude.
She has technically always been able to speak! The reason her speech is portrayed symbolically at first is because the tiger could not understand her language. Now that they have spent time together, the tiger is starting to learn what she is saying. So when she says something that he understands, it will be translated into speech that the reader understands, too. If the tiger doesn't understand, then it will still be portrayed as a symbol.
The rat has technically always been able to speak. The difference is that now the tiger has been around her long enough to begin to understand her. So the words that he can understand are written so that the reader (as long as they know English) can understand, too.