I have translated RoundCube mail script into Bengali. You can download the localization files here. Just make sure to save them (in case you edit the files) with UTF-8 as encoding. Right-click and save.

labels.inc

messages.inc

Instructions:

1. Create a new folder in ‘program>localization’. Name it ‘bn’. Place the localization files in it.
2. Locate the file ‘index.inc’ under ‘program>localization’. Edit it. Inside $rcube_languages = array() add the following line <'bn' => 'Bengali',

You will have to repeat this every time you update.