When did fluorescent lights become common in Europe?

In the mid-20th century, German electrical powerhouse, AEG commissioned scientist Dr. Charles G Hatay, a Hungarian, to improve on the design of the mercury-filled fluorescent tube. His work was later patented in Germany. Suffice it to say that a great number of brilliant minds from all over the world, influenced by each other's body of work, successfully led humanity from relative darkness into a brighter future. Many brilliant scientists do not, and often can not sell themselves or their discoveries. On the other hand, we have mega-corporations that only know how to sell themselves, a symbiotic relationship that should never overshadow scientific endeavors.

"Became common" is a tiny bit tricky. The light tubes themselves went into larger scale production in the 1880s.

But: Electricity itself wasn't that penetrating until after WW2. Then of course "Europe" is a somewhat big but incredibly fragmented place.

For cities —and with a focus on in Germany— we see:

The new tube quickly spread all over the world. In Germany the neon lights only caught on in the late 1920s since the direct current network held up their progress. But then flowing neon writing gradually replaced open-air advertis*m*nts that used combinations of different light bulbs. By this time a variety of noble gas advertis*m*nts were already installed in Tokyo. One of the largest Japanese daily papers had an advertising sign in front of the Imperial Palace, while in the entertainment quarter of Asakusa a sign proclaimed the ‘hq of Beef Stew’. In Mexico City, Havana, Melbourne, Wellington and Shanghai, neon firms were also in business by 1930.

— Christoph Ribbat: "Flackernde Moderne: Die Geschichte des Neonlichts", Franz Steiner: Stuttgart, 2011.

The above is translated from German, where 'neon-tubes' still is a technically incorrect but established synonym for 'fluorescent light tube', not necessarily – and in fact in most cases not — actually filled with neon. The mercury filled fluorescent tube being actually a mainly German invention from 1927 by Edmund Germer.

It seems that overall Europe moved indeed a bit faster than the US there, especially for indoor lighting applications:

The phrase "neon tubing" is commonly used as a generic term to include all low-pressure gas-filled tubes of the high-voltage type. Mercury vapor, helium, and many other gases are used, either alone or in combination, to produce colors different from neon's characteristic red. […]

During the twenties the desire for more distinctive colors led to the use of fluorescent glasses. By a proper selection of fluorescent materials, which were introduced into the glass itself, it was possible to transform some of the ultra- violet light given off by low-pressure mercury-vapor or other discharges into visible light of almost any desired color.

Until the end of the twenties neon-type tubing was employed almost exclusively for advertising and other outdoor illumination. It was not considered that the device was satisfactory for interior lighting because of the high voltages employed, the poor color quality of the light, and the expense of installation. Moreover, the efficiency of neon-type tubing was little if any greater than that of incandescent lighting. European producers, particularly in France, pioneered in the application of neon-tube lighting in the indoor field around 1930. Combinations of neon and mercury tubes gave a fairly balanced light output which was satisfactory in certain restaurant, store, and other interior applications. In the United States neon tubing was confined to the outdoor field for a much longer time. One reason for this was the restraint shown by General Electric and the Claude interests in not invading each other's market. Their previous unwritten understanding was put into more definite terms in 1938, with the signing of a twenty-year licensing agreement whereby Claude Neon was granted manufacturing and sales rights in the United States for made-to-order electric-discharge lamps for outdoor display and illumination only. General Electric evidently wished to hold down the cost of its failure to promote neon itself by limiting it to outdoor uses as much as possible.

Most further technical progress in neon-tube lighting was made in Europe, until after 1938.

— Arthur Aaron Bright: "The electric-lamp industry : technological change and economic development from 1800 to 1947", Macmillan: New York, London, 1949. (archive.org, p370)

Of course, for any sabotage action to be effective, such technology needs not to be really wide-spread, just used in a place where much damage could be associated with it when tampering with it. For "occupied Europe" however, any form of lighting would be quite reduced by the Luftwaffe's orders to often blackout much of the city lighting.