The alt tag for your language switcher is "nl". with their titles: hidden link
It seems that your theme is picking up the whole code as the anchor tag title.
If you examine your front-end code you will notice that the title is wrongly wrapped. (Please check the attached screenshot)
You can test that this is theme related by switching to a default theme such as twenty17 and examining the flags with it as well.
Standard code for language flag in my local environment