{{ if eq $currentPage . }} class="nav-link active" aria-current="page" {{ end }}
So full code is:
{{ $currentPage := . -}}
{{ range .Site.Menus.main -}}
<li class="nav-item">
<a class="nav-link" href="{{ .URL | relLangURL }}" {{ if eq $currentPage . }} class="nav-link active" aria-current="page" {{ end }}>{{ .Name }}</a>
</li>
{{ end -}}