Multilingual survey
From LimeSurvey Manual
How to hide the language switcher from the welcome page
In the case in which you decide to launch a multilingual survey, you will observe on the welcome page a drop-down menu that permits your respondents to select the language in which they wish to fill in your survey:
However, in certain scenarios you wish to hide the respective option. There are many ways in which you can hide the language switcher.
One method that we recommend is to edit the corresponding .twig file. Comment {% if aSurveyInfo.alanguageChanger.show == true %} and add {% if false == true %}
To do that, access Themes from the global Configuration menu.
Click on the Theme editor button to edit the desired theme:

Look on the left side of the screen for the language_changer.twig file.
Go to the line that contains the following function:
{% if aSurveyInfo.alanguageChanger.show == true %}
Comment it out and copy the following line below it:
{% if false == true %}
The line should look like this in the end:
Click on Copy to local theme and save changes to save your changes.
Now, the welcome page should look like this: