According to the answers to the question Excel to CSV with UTF8 encoding, Google Docs should save CSV properly, contrary to Excel, which destroys all characters that are not representable in the “ANSI” encoding being used. But maybe they changed this, or something wrong, or the analysis of the situation is incorrect.
For properly encoded Bangla (Bengali) processed in MS Office programs, there should be no need for any “Bangla fonts”, since the Arial Unicode MS font (shipped with Office) contains the Bangla characters. So is the data actually in some nonstandard encoding that relies on a specially encoded font? In that case, it should first be converted to Unicode, though possibly it can be somehow managed using programs that consistently use that specific font.
In Excel, when using Save As, you can select “Unicode text (*.txt)”. It saves the data as TSV (tab-separated values) in UTF-16 encoding. You may then need to convert it to use comma as separator instead of tab, and/or from UTF-16 to UTF-8. But this only works if the original data is properly encoded.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…