I read these books in English during the early 1990s. It was a series of two or possibly three books.
The general storyline is one of alien invasion in the future, with the protagonists being a group of human freedom fighters working for an organized resistance or underdog free government.
The protagonist group consisted of both men and women, and they were not fighting alone - one scene in the book consists of them running a training session for other fighters in an organized capacity, so there was some government remaining.
The protagonists group's spaceship was a big part of the series in its own right, similar in many aspects to how the Millenium Falcon is a big part of Star Wars - often mentioned, often worked into the plot.
The enemy alien race had multiple factions - one very militaristic, and one much kinder. The kinder faction often worked with the protagonists, while the militaristic faction was the one driving the war.
Major plot points from the books that I can remember include:
- During the training mission, one of the points was that the alien soldiers had personal force field generators which caused them to glow at night, and this was used as a training tool - the scenario was actually that the alien soldiers had anticipated an ambush and had disabled their force fields so as to catch attackers unawares.
- The very end of the last book sees the aliens on the verge of defeat, which is ultimately brought about by a female member of the kinder faction sacrificing herself by going into a "death trance" with the militaristic leader the target - this will kill them both, ending the war
There may have been a subplot involving one of the protagonists being marooned on a space colony of another set of aliens, and being taught to fight by their champion in a higher gravity part of the colony - I may be misremembering this from another book series however.