An old dog barks not in vain.
Meaning: Proverb intended to signal the seriousness of a situation. For an old and experienced dog would not - figuratively speaking - waste its strength on useless barking.
Ein alter Hund bellt nicht umsonst. 
Ega vana koer valet ei haugu. 
En gammel hund gøer ikke forgæves. 
En gammal hund skäller inte förgäves. 
Vanha koira ei hauku turhaan. 
Senas šuo loja ne veltui. 
Vecs suns ne velti rej.