This is not software problem this is a font issue. We use ubuntu fonts and this letter does not support bold in Ubuntu fonts, you can change it to Arial for example and it will be displayed properly. 

Thanks, but why then is it possible to display accented letters correctly on Ubuntu font page: http://font.ubuntu.com/ ?

Give it a try: enter the word «VÉRIFICATION», select «Weight: Bold» and everything will appear exactly as it should!

Are there some files that could be updated in Oxwall to make this work using the Ubuntu font?

Many thanks!

Ubuntubold which is not Ubuntu font in bold weight does not support such characters, which is why you need you need to use another font for example Ubuntu

you can redefine this by using Google fonts in the Theme customizer: 

@import url(http://fonts.googleapis.com/css?family=Ubuntu:400,700&subset=latin,greek-ext,greek,latin-ext,cyrillic,cyrillic-ext);
.ow_box_cap h3, .ow_box_cap_empty h3 {font-family: 'Ubuntu';font-weight: bold;}

or the easiest way is to use Arial font, which is presented in all operating systems. 

.ow_box_cap h3, .ow_box_cap_empty h3 {font-family: 'Ubuntu';font-weight: bold;}