This online tool accurately inserts the historical long S character ( ſ ) back into text in order to make it appear as if it were written before the 20th century.
English, French, German, Spanish and Italian are supported.
This is a JavaScript version of my original Python code found at py-long-s.
Thank you Andrew West of the TeX Users Group for the documentation found under The Rules for Long S, which was fundamental in writing the conversion functions for English, French, Spanish, and Italian.