Humans are vulnerable in water. Beaches have red flags; swimming pools have flashy warning signs to remind us of our vulnerability when we just want to cool down in the midst of a searing heatwave. Pool rules are essential, especially when children are around, or tourists who don’t know about the local safety measures. With pictograms and whistling lifeguards, swimming pools usually manage to communicate danger without requiring visitors to pass a language test at the entrance. Until now, that is.

In the eastern German city of Halle, a public swimming lake turned away visitors who did not speak German during one of the hottest weeks of the year. The operator of the Heidebad natural pool at Heidesee lake, Mathias Nobel, argued that people without sufficient language skills may fail to understand the rules and thereby put themselves at risk. He said that as a trained lifeguard, he recently had to rescue a small child without armbands from the water, since the lake, a flooded former opencast mine, had a steeply sloping shoreline.

The new language requirement may therefore sound like a concern for public safety to some. To others, and to me, it sounds suspiciously like something else.

While it did not take a definitive position on the case, a spokesperson for Germany’s Federal Anti-Discrimination Agency said that denying access to a pool over the lack of German language skills could legally constitute discrimination on the basis of race or ethnicity. Nobel denied the measure was racist or xenophobic.

But if safety rules were genuinely the concern at Heidebad, the solutions are embarrassingly obvious. Even the city of Halle has urged the operator to withdraw the rule and pointed to alternative safety measures, including pictograms and multilingual information. The city itself has argued that ensuring safety does not justify excluding entire groups of people.

That raises an uncomfortable question. If more inclusive alternatives are readily available, why was exclusion chosen first?

A swimming pool is not just a place of recreation. When temperatures climb above 35C, access to water becomes a matter of public health. To deny entry to people because they are not fluent in German is not a neutral act. It is a decision about whose wellbeing and health matters.

This incident, and the political commotion it has triggered, arrive at a particularly troubling moment. Halle is located in Saxony-Anhalt, where campaigning has begun ahead of state elections in September. The far-right Alternative für Deutschland (AfD) is expected to dominate the contest, and polls suggest it’s on the verge of winning a majority of seats in the state assembly. For the past decade, migration has been the central theme of every political debate in eastern Germany. The distinction between “citizens” and “foreigners”, between those who belong and those who are merely tolerated, is increasingly drawn at the centre of public life.

So it’s not surprising that the pool’s entrance policy was instantly supported by the AfD. On Tuesday, the party drew up its very own swimming pool sign, stating: “Those who don’t understand German, stay out.” While the pool operator may argue that his ban was also for the safety of non-German speakers, the AfD unashamedly presents them as the danger. If the dog whistle wasn’t loud enough, the sign is presented in a montage next to three Middle Eastern men. Get it?

The city of Halle has a recent and painful history of violence against marginalised groups. In 2019, a far-right extremist attempted to carry out a massacre at a synagogue on Yom Kippur. Failing to enter the building, he murdered two people: one outside the synagogue and another at a nearby kebab shop. The attack was shocking, but it also exposed the deadly consequences of an atmosphere in which certain groups are continuously portrayed as burdens.

This context colours the Heidebad incident in darker undertones. As German history has shown, a society rarely leaps from peaceful coexistence to violence in a single bound. Countless small acts of exclusion erode our sense of community, of a shared public life, until they normalise discrimination as common sense.

For years, public discourse in Germany has repeatedly transformed pools into symbolic battlegrounds over migration and integration. In 2016, a swimming pool in Bornheim imposed a temporary ban on male refugees after allegations of sexual harassment. Critics warned at the time that such policies punished innocent people while justifying racial profiling.

Every summer, isolated incidents involving migrants are blown up by the press and social media into national debates. The idea that some people require special surveillance and restrictions keeps returning in different forms. And every summer, there are enough Germans insisting that what they are witnessing has nothing to do with racism.

With its “German speakers only” fake sign, the AfD makes it clear that the case in Halle was never really about safety. The debate was about who German institutions are willing to make things harder for – since installing multilingual signs requires effort, but turning away migrants requires only suspicion.

You can imagine the scene at the entrance of a crowded outdoor pool during a heatwave. Some people are waved through. Others are interrogated as if they are at a border checkpoint. They are asked to prove that they belong, and that they have the right to cool down. This may make you shudder. A growing number of German voters will picture this and nod in approval at what is to them a desirable future for their country.

• Fatma Aydemir is a Berlin-based author, novelist, playwright and a Guardian Europe columnist