There are many English-English dictionaries out there for mobiles (Like AED, MSDict etc) but it is hard to find a good English-Hindi (or English to other local language) dictionary. Being from Hindi spoken region of India I too missed this utility in my mobile until I found following two very useful dictionaries:
1. MIDlet English-Hindi Dictionary (free Open source app): A sourceforge project app (Constantly under development). Contains almost all common and not so common words. Each word is explained in detail along with its use in sentences. However Hindi words look malformed at many places. Download from here.
2. English-Hindi (Indic) dictionary: It is a shareware MIDlet (Java) dictionary by iFreeMobile. Contains a large word database and is easier to use (Suggest words when typed in the “Find” bar). Hindi words are well formed and are easily readable. Download from here.
Both the dictionaries are Java apps hence compatible with almost all phones supporting Java MIDlets (Including Nokia S40, Symbian S60v2, v3, v5 phones, most new Samsung, Motorola, Sony, LG mobiles). I have tested these in Nokia 6303, Nokia N70, Nokia N95 and Nokia 5233. Both work best at 240*320 screen resolution. Try yourself and share your views.