Hello @ll,
while configuring LO
Version: 5.1.0.0.alpha1+
Build ID: 9be553f4c61f220ebbe212dc76cb9cce4ae1c106
TinderBox: Linux-rpm_deb-x86_64@46-TDF, Branch:master, Time: 2015-07-13_22:45:19
Locale: de-DE (de_DE.UTF-8)
(parallel installed, following the instructions from https://wiki.documentfoundation.org/Installing_in_parallel/Linux)
on Debian Testing AMD64, I discovered the following:

1. Start LO
2. Go to "Tools - Options – LibreOfficeDev – Fonts"
3. Mark "Apply replacement table"
4. Enter e.g. "Arial" in "Font"
5. Enter e.g. "Liberation Sans" in "Replace with"
6. Try to enable it either on "Always" or "Screen only"

On my system, nothing happens ... :( I am not able to enable font replacement in this dialogue ... :(

This works in LO
Version: 4.4.4.3
Build-ID: 40m0(Build:3)
Gebietsschema: de_DE.UTF-8
(Debian's version with Germanophone lang- as well as helppack)

as well as with

Version: 5.0.1.0.0+
Build ID: b0153639c17d40061480a7bbde11fa0249e3051f
TinderBox: Linux-rpm_deb-x86_64@46-TDF, Branch:libreoffice-5-0, Time: 2015-07-14_03:50:43
Locale: de-DE (de_DE.UTF-8)
(also parallel installed)

and Version: 5.0.0.3
Build-ID: f79b5ba13f5e6cbad23f8038060e556217e66632
Gebietsschema: de-DE (de_DE.UTF-8)
(the same as before ... ;) )

If you need any further information, feel free to ask :)
Have a nice day
Thomas.