Creating the vectors for each letter was done in Adobe Illustrator, and using those vectors to fill each glyph to create the actual font was done in Fontlab Studio 5 http://www.adobe.com/nl/products/illustrator.html https://www.fontlab.com/font-editor/fontlab-studio/
I did it this way since I had to trace each individual letter, and it was easier doing the tracing in Illustrator since I know that program, and I'm able to set a picture (the scan of my written letters) as a background image. Then I copied the outline vectors over to Fontlab.
Font software is very accessible. I started with FontLabs and a beginner's manual for FontLab called Practical Font Design.
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Fonts are an unusual area of copyright. See this short article which covers some relevant issues.
Because of the huge differences between the English alphabet (also known as the 'Latin' alphabet) and the Bangla alphabet, I would imagine that even if you are inspired by an existing font, your own font is ultimately going to be highly original.
Hope this helps! :-)
Happy to help! I don’t know the first thing about world building, but I imagine it is a huge amount of work on itself even when you don’t design a font for it :)
Regarding other font editors, the most popular ones are Glyphs (Mac only) and Fontlab Studio (Mac or PC). They are much easier to use than Fontforge, but they are not on the cheap side. That being said, the calt
substitutions need to be manually written on all font editors, so you would need some basic knowledge of OpenType feature code anyways. Feel free to DM me in case you need any help to write it.