I assume we are talking about this image:
http://bits.wikimedia.org/w/extensions-1.19/MobileFrontend/stylesheets/images/logo-copyright-en.png

The logo could 404 by default meaning the installer would need to set it up themselves.
Alternatively instead of 404ing the watermark could be a more neutral mediawiki logo by default with instructions in the README on how to change.

We could change the alt text to derive itself from the wiki name rather than a language file
Thoughts?

Were not using the logo in any other language. It's purely text on all others.

We should treat the image the same as setDefaultLogo in MobileFrontend.php

e.g. $wgMobileFrontendWatermark should be used for the image in the footer
The watermark should also allow for language
e.g. $wgMobileFrontendLogo = '/MobileFrontend/stylesheets/images/watermark-$1.png';
on a French language site would look for watermark-fr.png
If $wgMobileFrontendLogo doesn't contain $1 it will use the same watermark for all languages

We would also need to check for the existence of the watermark image file rather than the current test of
if ( $wgLanguageCode === 'en' ) so that the user has more control over when the image can appear

https://gerrit.wikimedia.org/r/gitweb?p=mediawiki/extensions/MobileFrontend.git;a=blob;f=skins/SkinMobile.php;h=f71000fffd634f030ae735165deaf0e0619df29e;hb=master#l144

I don't feel too comfortable with this code to do this. Any volunteers want to submit a patch?

fixed in https://gerrit.wikimedia.org/r/7058
If/when merged we will need to update http://www.mediawiki.org/wiki/Extension:MobileFrontend#Configuration_settings