Can you reduce every decidable language to a regular language?
In fact, every non-trivial language is $\text{R}$-hard. That is, every decidable language is reducible to every non-trivial language. Indeed, let $A$ be a decidable language, and let $B$ be a non-trivial language. A reduction from $A$ to $B$ operates as follows. On input $x$, check whether $x\in A$ (this can be done as $A$ is decidable), then:

if $x \in A$, the reduction outputs $y_{in}$
if $x\notin A$, the reduction outputs $y_{out}$

where $y_{in}$ and $y_{out}$ are constant words in $B$ and $\overline{B}$, respectively.