The move piles pressure on the Irish budget airline after pilots in Ireland, Sweden and Belgium also vowed to strike on August 10, promising major disruptions for thousands of holiday makers.
Europe's second biggest airline has been grappling with staff unrest since it recognised trade unions for the first time in December 2017, in a bid to ward off widespread strikes over the Christmas period.
But unions say their calls for better wages and fairer contracts have gone unheard, and no progress has been made despite months of negotiations.
Germany's Cockpit union said Ryanair management had failed to respond to a yesterday deadline for an improved offer, leaving them with no choice to go on strike.
"There can be no improvements without increasing staff costs," Locher said, but this has been "categorically" rejected by Ryanair.
"At the same time, Ryanair has at no stage signalled where there might be leeway to find solutions. Ryanair alone is responsible for the escalation we are now seeing."
Ryanair pilots have long claimed that they earn less than their counterparts at rival budget airlines.
Another big complaint of workers based in countries other than Ireland is the fact that Ryanair employs them under Irish legislation, arguing most of its employees work on board Irish planes and are therefore covered by Irish law.
But staff counter that this creates huge insecurity for them, blocking their access to state benefits in their country.
Ryanair is due to hold its own press conference in Frankfurt at 1:30 pm (1130 GMT).
The airline was already hit by a round of strikes last month affecting 600 flights in Belgium, Ireland, Italy, Portugal and Spain.
It said the 100,000 affected passengers had all been put on alternative flights or would receive refunds.
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