By default, Limesurvey provides the follow text elements for the surveys - Survey title, Description, Welcome message, End message etc, which I can use in my template with tags like {SURVEYNAME}, {SURVEYDESCRIPTION}, {WELCOME} etc.
Is it possible to add my own custom field, which I can then use in the template? I need to do it this way because I need to have the text translatable, and present on every page.
You can not add a custom replacement with the current version of LimeSurvey. And your LimeSurvey version seems outdated. But LS includes jquery, therefore it's easy to move some element from a place to elsewhere.
Quick example:
<p>Here is your description</p>
<div style='display:none'>
<label for='languagechanger' id='labellang'>Here is the new label for language</label>
</div>
<script>
$(function() {
$("#labellang").insertAfter("#languagechanger")
});
</script>
A PHP solution, hacking LimeSurvey code, should be placed at https://github.com/LimeSurvey/LimeSurvey/blob/master/application/helpers/replacements_helper.php#L814