AvailableDescriptionLanguages: Use one string for all iterations

AvailableDescriptionLanguages: Use one string for all iterations

Do not create strings within the loop, that creates one string
per language and does more work than needed. Instead, reserve
enough space at the beginning and assign the prefix, and then
resize and append inside the loop.

Also call exists with the string itself instead of the c_str(),
this means that the lookup uses the size information in the
string now and does not have to call strlen() on it.