Skype has many localizations, both official and community-based. But most of them are only available on Windows (via customized .lang files) but although I've looked, there is no way of using the .lang files we have created on MacOS or Linux. That's ... not helpful.
Regarding your question: The Skype application on Windows, Mac and Linux are built using completely different technology to achieve best performance and system integration on each platform. This also means that the text files cannot be shared across plattforms. Additionally terminology differs per platfrom (Windows taskbar vs MacOS doc, Windows systray vs. Linux menubar).
Any language your are missing on a certain platform?
Yes but those are minor difference which could be adapted easily. Well, the easiest thing would be if there was one large file which contained the strings for all OS and the software in question just pulls the system specific ones.
It's Scottish Gaelic on OSX that people keep asking me for.
While we're on the topic, the .lang format isn't great to work with, I had to convert it to a .po file manually to be able to use a translation memory, there's a lot of recurring stuff but no TM I'm familiar with handles .lang. If the file for MacOSX needs a re-translation from scratch, then some format which can be handled by a TM would be great as it would really save time and improve consistency.
You can translate .po and .lang files with .po and .ini file filters by CAT tool OmegaT supports TM. Gettext .po files can be obtained from .ts by ts2po tool from translate-toolkit package on Linux.
And you can use transifex.com translation platform with .po and .lang files without any converting (maybe only UTF16←→UTF8 converting).