The last version of @roytam1's FxESR 45.9.x fork that would accept the official Mozilla Fx 45.9.0 language packs,
https://ftp.mozilla.org/pub/firefox/releases/45.9.0esr/win32/xpi/
is package "firefox-45.9.29-20201010-7391af2bb-win32-sse.7z";
later versions have become incompatible with the original language packs (LPs hereafter...).
In said compatible build, browse to the LP you want to install (e.g. Greek = el.xpi) and click on the link;
allow the site to install extensions, then click Install;
after the installation has been completed, open about:config and locate pref
general.useragent.locale
Change its value from the default (en-US) to the one matching the freshly installed LP (e.g. el);
restart the browser and voila:
OTOH, if you wish to move to a more recent Fx45esr version, you'd have to use English exclusively, or adapt yourself the existing LPs to accommodate the locale changes introduced by the "upstream" Roy follows to produce his fork...