Is endoplasmic reticulum involved in transcription or translation?

Translation (As ribosomes are present on RER)

Strictly speaking, it isn't involved in both.
But since ribosomes are found on endoplasmic reticulum, you can say it's more or less related to the translation process.

It doesn't play much of a role in transcripton since the mRNA transcript is assembled in the nucleus of a cell, but can play a role in translation. Some proteins need to be exported to the cell membrane or even outside the cell. In these cases, the proteins contain a signal peptide at the N-terminal. While a protein is being translated, it halts when the signal peptide binds to the signal recognition particle (SRP). The SRP binding to the signal peptide halts translation, and allows the entry of the protein into the ER, so it can be modified and packaged and exported.

-Manpreet Basuita

More translation, but both.