High gothic is very similar to latin in the early middle ages- a language spoken by all members of the upper classes and fairly standard regardless of where the speaker is from. So a medieval noble/scholar/priest/merchant would (probably) know any nessesary local languages, to another noble/scholar/priest/merchant they would speak latin, particularly if the other party was from a different geographical location. In a similar way High Gothic will be the language generally used for religious ritual and major works of scholarship (NOT science so much, however- that's more the Admech's domain).
Low gothic I understand as being a debased (but very much distinct) form of high gothic. Remember that in the background, guardsmen from different regiments are shown as being able to converse fairly easily in Low Gothic. Certainly there will be a lot of local variation in terms of slang word, figures of speech and accent, but the grammar and core vocabulary will probably be very consistant. Remember that in many cases (the guard and navy, trading vessals, and much of the Departmento Munitorum) there is a lot of contact across the Imperium, and in many cases Low Gothic will be the only language two livestock traders or navy crew will have. The imperium being what it is, I imagine most planets make an active effort to keep 'their' Low Gothic in line with the standard.
Many planets will have low gothic as their standard language, but many more will have another language specific to them as well. This probably happens as much on Death Worlds and frontier worlds as much as low-tech feral/feudal worlds. In many cases this will be based (losely) on low gothic, with significant changes in grammar, vocabulary and intonation. Similarly, some isolated worlds will have their language based on High Gothic, other minor dialects or possibly even Xenos languages, if you trace it back far enough. On many such planets there will be several distinct languages for different geographical regions, castes or even social situations.
All of ^that^ is my own oppinion, but certainly not cannon...
As for the orks, the above post has nailed it, IMHO.