Some languages can't be selected because Symfony distribution does not include the corresponding culture info file. This is also happening with other languages like Achinese. These culture info files include information like the type of calendar used and other culture-specific details used for common operations such as formatting dates, currency, etc...
The easiest solution is to create a culture info file for Scottish Gaelic. Its language code is "gd". Follow these instructions:
1) Open your ICA-AtoM directory
2) Go to vendor/symfony/lib/i18n/data
3) Create a new
plain text file called "gd.dat" and include the the text below inside:
s:0:"";
(or you can create it from another file like es_ES.dat)
Then, go to ICA-AtoM site and try to add your language again, it should work now.
If you are trying to guess another culture name, the format followed is "<languagecode>_<country/regioncode>", where <languagecode> is a lowercase two-letter code derived from ISO 639 codes. You can find a full list of the ISO-639 codes at
http://www.ics.uci.edu/pub/ietf/http/related/iso639.txt