markus, have a look how I fixed it for question text:
https://github.com/LimeSurvey/LimeSurvey/commit/cd2341a82471975d376abf0c00db2612fd16eca2

First we check if the srting contains a '{' to avoid useless logic:
https://github.com/LimeSurvey/LimeSurvey/blob/93eda227d917a0c142d524fe91949eb951d0bc36/application/helpers/SurveyRuntimeHelper.php#L374-L380

So you can move this test in a function out of surveyRuntime

Then, if it has a '{', it will call the function getStandardsReplacementFields to generate the Standard Replacement Fields and their current value:
https://github.com/LimeSurvey/LimeSurvey/blob/93eda227d917a0c142d524fe91949eb951d0bc36/application/helpers/SurveyRuntimeHelper.php#L377

The function getStandardsReplacementFields is defined here, based on the old templatereplace (so if some standard variables are missing, you should find them in templatereplace):
https://github.com/LimeSurvey/LimeSurvey/blob/93eda227d917a0c142d524fe91949eb951d0bc36/application/helpers/replacements_helper.php#L362-L481
https://github.com/LimeSurvey/LimeSurvey/blob/93eda227d917a0c142d524fe91949eb951d0bc36/application/helpers/replacements_helper.php#L34

THen finally the replacement is done via the EM here:
https://github.com/LimeSurvey/LimeSurvey/blob/93eda227d917a0c142d524fe91949eb951d0bc36/application/helpers/SurveyRuntimeHelper.php#L386

So globally, getStandardsReplacementFields should be called only once (or it should use a cache), and then you can apply it doing:

LimeExpressionManager::ProcessString("HERE THE STRING WHITHT SOME VARIABLES ", $qa[4], $aStandardsReplacementFields, 3, 1, false, true, false);

Btw:
https://github.com/LimeSurvey/LimeSurvey/blob/master/themes/survey/vanilla/views/subviews/content/submit.twig#L71

{# NOTE: not very clear what the old replacement keyword {URL} was doing #}

Now I know ^^

https://github.com/LimeSurvey/LimeSurvey/commit/4a8c0285161aa56c5dcec9f72e3bb9467d0e1b9a

Fixed in 3.1.1.

still not fixed in Version 3.1.1+180130
it still shows {variable} instead of the variable's content

LSS attached (LSA doesn't work, will create another bug report for that)

I raised a new ticket as I'm not sure if you're following "closed" bugs :)
https://bugs.limesurvey.org/view.php?id=13280