It may indeed be difficult to translate, since I have not found it in any available dictionary. It is not mentioned in on-line NATO terminology glossaries, I have found it neither in my WNT (Science and Technical Publishing House) dictionaries, nor in a few of other technical dictionaries. However, it must be translated somehow. When analysing single items of the expression, one may assume that the term is related to fuel, capacity (meaning volume? power rating? throughput?) and loanding/unloading. How then we are expected to put it together in a logical way in Polish? Firstly, let's find the term in the context. The query "fuell off-load capacity" in Google and the number of results show that it is quite a rare term — according to what the searcher suggests, there are no more than ~500 results, but actually, we have only 2 pages with links to websites with this expression. With such a number of results, looking for bilingual sources containing both an English and a Polish version of the term would not help. What is more, not all bilingual materials provided in the Internet are reliable. One of Boeing articles says as follows:
Fuel Off-Load Capacity of 20,000 gallons (75,710 liters)
There was little context — firstly, the line is in the table, and secondly, this not a sentence at all. This fragment says only that the "Fuell off-load capacity" value is measured in gallons or litres. Better this than nothing...
The next example was found at www.aerodefensetech.com.
The Airbus DS MRTT Voyager has settled down to being a very capable and mature military air asset, and a leader in its field, offering more usable cabin space and fuel off-load capacity, endurance and a more modern airframe than rivals. (Richard Gardner)
Based on this, you can boldly assume that our term is a desirable technical parameter of an aircraft. Another important bit of information here is that this parameter is particularly relevant for Airbus DS MRTT Voyager, which is a military tanker. Using this knowledge for further research, you can obtain more details. For example the following synonymous expressions "tanker off-load capability" or just "fuel off-load(s)". Moreover, at this stage, we already know, based on many topic-related articles, that "offloading" in this context refers to the operation of unloading fuel from the tanker to another aircraft, of feeding the other aircraft. The gathered information sufficiently ensures the technical term "fuel off-load capability", provided it occurs in aicraft-related specialised writings, may, be translated into Polish descriptively as: ilość paliwa, którą można przepompować z samolotu tankowca (which is by the way confirmed by English-Russian aviation and space dictionary (M.: Military publishing house USSR Ministry of Defence. edited by M. Murashkevich. 1974.). The descriptive translation can be also be shortened, leaving the original a little bit aside, but keeping the meaning: e.g. "Pojemność przeładunkowa tankowca". Such a meaning was also confirmed by one of the employees working in the aviation field.
Fuel off-load capacity/capability — Pojemność przeładunkowa tankowca
The term also takes many variations. Instead of fuel sometimes there is tanker, off-load occurs also as offload and offloading, and capacity becomes from time to time capability.