Hi Andreas,
I’ve done it on my site by adding it to the .languagewrapper .language.active a part of the footer code injection. As you can see below I’ve used specific weights, but you can use normal and bold for fonts that aren’t available in all weights.

.languagewrapper .language.active a {
color:black;
font-weight: 400;
}

.languagewrapper .language a {
padding:5px 5px;
text-decoration: none;
color:black;
font-family: “Oswald”!important;
font-weight: 300;
}