FreekMarker software from which *.ftl templates originate support export to several formats, including HTML so this software should be your first, and most likely also only software capable of FTL to HTML conversion.
Convert FreeMarker template file to HyperText Markup Language web page.
We found one software available in the database for this conversion.
Microsoft Windows software - convert ftl to html on Windows
A template engine for generating texts
Convert from ftl file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.