As mentioned in the title, I'm currently facing the problem that something with/in Bootstrap might break the usual way of using custom fonts.
I add
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
right in the beginning and
<!-- Custom CSS Import -->
<SharePoint:CssRegistration Name="<% $SPUrl:~sitecollection/_catalogs/masterpage/WTE/Styles/bootstrap.min.css %>" runat="server" />
<SharePoint:CssRegistration Name="<% $SPUrl:~sitecollection/_catalogs/masterpage/WTE/Styles/bootstrap-custom.css %>" runat="server" />
in the end of the part.
Additionally
<!-- jQuery (necessary for Bootstrap's JavaScript plugins) -->
<script type="text/javascript" src="<asp:Literal runat='server' Text='<%$SPUrl:~sitecollection/_catalogs/masterpage/WTE/Scripts/jquery-3.3.1.min.js%>' />" ></script>
<!-- Include all compiled plugins (below), or include individual files as needed -->
<script type="text/javascript" src="<asp:Literal runat='server' Text='<%$SPUrl:~sitecollection/_catalogs/masterpage/WTE/Scripts/bootstrap.min.js%>' />" ></script>
<script type="text/javascript" src="<asp:Literal runat='server' Text='<%$SPUrl:~sitecollection/_catalogs/masterpage/WTE/Scripts/bootstrap-custom.js%>' />" ></script>
is added right before the closing body tag. The rest is the simple seatle master page. What am I missing? In seatle master page alone it works just like a charm. So the font itself is ready to use and my spfont should be without any mistakes.