Marije,
That is an esri default bundle string so you have to change it using this code:
postMixInProperties: function() {
this.inherited(arguments);
jsapiBundle.planRoutesTool.maxPtsRoute = "Maximum number of stops per walking route";
lang.mixin(this.nls, jsapiBundle.analysisTools);
lang.mixin(this.nls, window.jimuNls.common);
this.nls.toolNotAvailable = settingBundle.toolNotAvailable;
},