Non "Hydropneumatic"
There are other forms of accumulator used for fluid power hydraulic purposes. For example, coil spring plus sealed piston; though these are less popular. Therefore a hydraulic accumulator is not necessarily a hydropneumatic accumulator.
American English
In American English language, the search is for hydraulic accumulators.
American English outside the US
In American English outside the USA, the search is for hydraulic accumulator, the singular.
UK English
In UK English the search in the UK, is for hydropneumatic accumulators, plural.
UK English outside UK
In UK English the search outside the UK, is for hydropneumatic accumulator, singular.
Germany
In Germany the search is for Druckspeicher (pressure store), singular and plural; and some times hydropneumatischer Akkumulator.
French
In French the search is for accumulateur hydropneumatique, singular.
Spanish
In Spanish the search is for accumulator hidrapneumatic.
Read more about this topic: Hydropneumatic Device