The qualculate sources include definitions of additional units, functions, etc.
which libqalculate tries to load on startup. These are not hard requirements.

libqalculate is e.g. used but the Plasma calculator krunner. If the qualculate
package is installed (which provides a command line calculator, and *also* the
xml data files), the following expression in KRunner works:

= 3 * 10^8 m/s * 24 * 60 * 60 * 365 s / lightyears
-> 1,000007349054008

Without qalculate installed, krunner issues some warnings:
---
I/O warning : failed to load external entity
"/usr/share/qalculate/prefixes.xml"
I/O warning : failed to load external entity
"/usr/share/qalculate/currencies.xml"
I/O warning : failed to load external entity "/usr/share/qalculate/units.xml"
I/O warning : failed to load external entity
"/usr/share/qalculate/functions.xml"
I/O warning : failed to load external entity
"/usr/share/qalculate/datasets.xml"
I/O warning : failed to load external entity
"/usr/share/qalculate/variables.xml"
I/O warning : failed to load external entity
"/usr/share/qalculate/currencies.xml"
I/O warning : failed to load external entity
"/usr/share/qalculate/eurofxref-daily.xml"
---

-> (9460800000000000 "m" "s") / ("s" * li("g" "h" "t" * year("s")))
i.e. it fails to parse the expression completely

simple expressions (i.e without e.g. units) work with or without the data sets:
= 100 * 20 + 1
-> 2001