In the file '/course/format/templates/local/content/cm/cmname.mustache' there this the markup:

<img src="{{{icon}}}" class="activityicon " alt="{{modname}} icon">

The 'alt' attribute is used to provide a human language alternative description of the image, and yet here there are hard coded English words used, so 'icon' and the templated module name, which will also be English, such as 'forum'.  For example, 'icon' in Welsh is 'eicon'.  Therefore this needs fixing to something like:

<img src="{{{icon}}}" class="activityicon" alt="{{pluginname}}">

where the templated 'pluginname' will change as the language changes.

Note: Tracker text editor may have messed with the curly brackets.