In this moment Bengali language is not supported in symbian belle.
Only officially supported languages are
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Arabic (Arabic),
Basque (Latin),
Bulgarian (Cyrillic),
Catalan (Latin),
Chinese [PRC] (Simplified Chinese),
Chinese [Hong Kong] (Traditional Chinese),
Chinese [Taiwan] (Traditional Chinese),
Croatian (Latin),
Czech (Latin),
Danish (Latin),
Dutch (Latin),
English [UK] (Latin),
English [US] (Latin),
Estonian (Latin),
Finnish (Latin),
French (Latin),
French [Canadian] (Latin),
Galician (Latin),
German (Latin),
Greek (Greek),
Hebrew (Hebrew),
Hindi (Hi),
Hungarian (Latin),
Icelandic (Latin),
Indonesian [Bahasa Indonesia] (Latin),
Italian (Latin),
Kazakh (Cyrillic),
Latvian (Latin),
Lithuanian (Latin),
Malay [Bahasa Malaysia] (Latin),
Norwegian (Latin),
Persian [Farsi] (Arabic),
Polish (Latin),
Portuguese (Latin),
Portuguese [Brazilian] (Latin),
Romanian [Romania] (Latin),
Russian (Cyrillic),
Serbian (Latin),
Slovak (Latin),
Slovene (Latin),
Spanish (Latin),
Spanish [Latin America] (Latin),
Swedish (Latin),
Tagalog [Filipino] (Latin),
Thai (Thai),
Turkish (Latin),
Ukrainian (Cyrillic),
Urdu (Arabic),
Vietnamese (Latin).
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But u can download some 3rd party softwares from Nokia Store to support many other languages but not bengali..
I also having trouble with bengali SMS :\
Anyway hope them to include it in future os