[Show thumbnails] label not translating (Polylang & NextGen)

We are using Polylang as our multilingual setup for our web site. Whilst we are able to translate the strings for the gallery titles and captions, there is no string translation for the labels. I have found in the past that by using the plug-in Codestyling Localization, we can ensure that there are .po and .mo files that relate to the languages needed and this is the case here too. Checking the .mo and .po files, there are translations for this to Spanish [Mostrar miniaturas] and they appear in the .mo and .po files. However, when switching between the languages on the web site, these are not displaying correctly – on the Spanish version of the page, it displays [Show thumbnails] instead of [Mostrar miniaturas]

http://guardian-angels.es/new/en/events-gallery-2014/

and

http://guardian-angels.es/new/es/galeria-de-eventos-ano-2014/

We are using a single gallery for both languages and inserting the shortcode with the gallery id. Could a method for inserting the locale be added to the shortcode perhaps?

Thanks

Steve

https://wordpress.org/plugins/nextgen-gallery/