How to save a track in FIT format

[routeconverter]

I am surprised that RouteConverter uses GPS Babel.

RouteConverter uses GPSBabel for a few rare and binary formats. It used GPSBabel for Garmin FIT but since June 2018 RouteConverter contains native Java support for Garmin FIT.

I use gpsbabel 1.5.4, the last version, on my Linux machine. It cannot read Garmin FIT version 2 format, only version 1, while RouteConverter can read both formats

Version 2 was the reason to implement Garmin FIT support in Java. But since nobody asked, I've never implemented write support for it.

That is why I think RouteConverter can read more formats than GPS Babel.

You're right. RouteConverter uses GPSBabel for 16 formats and can read 68 formats in Java.