I downloaded a .po file following the steps at:
manual.limesurvey.org/Translating_LimeSurvey/en

I downloaded the Dutch version at:
translate.limesurvey.org/projects/limesurvey-4/

The filename is: limesurvey-4-nl.po .
I edited this file with Poedit. I changed 'Geen antwoord' (= no answer) in 'Nvt'.

After saving I obtained a .mo file: limesurvey-4-nl.mo .
I copied this file to: /usr/share/locale/nl/LC_MESSAGES/

Before I copied, there was no limesurvey mo file in this folder.

Then I opened LimeSurvey, and made an array question optional (instead of obligated).
I saved the changes and viewed the question.
Unfortunately, the result is still a column with 'Geen antwoord'.

So it looks if the .mo file is not taken into account.
Why is this not the case?

Do I need to do some extra things?

Hi,
I had a look into my installation.
And there is a file called "nl.mo" in "limesurvey/locale/nl".

I do not see that "/usr/share/locale/nl/LC_MESSAGES/" is a folder inside your LimeSurvey installation.

So you shouldn't rename the file, even if the downloaded file is called "limesurvey-4-..."
You have to upload the "nl.mo":

Furthermore try to clear the asset cache.

Joffm

Using Ubuntu 20.04 I found nl.mo in:

/var/www/html/limesurvey/locale/nl

How can I clear the asset cache?

Just empty this folder:

/var/www/html/limesurvey/tmp/assets

?

Exactly there is it.

Empty cache in "Configuration / Global Settings / General"

Joffm

Here you spoke for assets not cache …
It must be inside .tmp/runtime

But : check with debug = 2 before.

Please keep in mind that in LS 4.x a bug prevents some of the strings to be translated. Not sure if this is the case for your string, but it is possible.