It could have been tricky to represent to the audience the (perhaps) four different languages (French, German, Italian and English) spoken by the characters. The programme uses the device of representing each language with English spoken in a theatrical foreign accent.
For example, an exchange between French-speaking characters, conducted in English with a French accent, is totally incomprehensible to the British airmen until Michelle (the only French character who speaks English) switches to Bertie Wooster-esque "top hole, old chap" style banter in an upper-class English accent. The British undercover Officer Crabtree, in the permanent disguise of a French-speaking gendarme, speaks abominable French. His (presumed) mangling of French vowels is represented by similarly distorted English, most famously his customary greeting catchphrase of "Good moaning"; many of his distortions come out as innuendoes, such as "I was pissing by the door, and I thought I would drip in". The Germans, generally, speak in a more guttural way than the French.
Bertorelli, the Italian captain, speaks in a nasal tone, generally adding an "-a" at the end of certain words: for instance in his catchphrase, "What a mistake-a to make-a!". Other examples included "We drop-a the bolls", "I kiss-a your hand-a". In spite of the difficulties in communicating with the British characters, the French, Germans, and Italians all understand each other perfectly, the implication apparently being that they all understand French (and Bertorelli understands German spoken when no French are present) which they use when talking to one another, but in which their own accents remain evident.
When one particular plan calls for Herr Flick and Von Smallhausen to impersonate British airmen, a gramophone record is used to learn the 'nuances' of English. This essentially consists of the non-word sounds suitably voiced with the signature 'upper-class English accent' employed in the programme.
In one episode, René is actually forced to speak German. His voice is noticeably more high-pitched, which may be a gag concerning the way the Germans talk.
The last few series introduced a new gag, where Colonel von Strohm and Lieutenant Gruber are put in situations where they have to speak in a strange manner. In one episode the two try to learn Spanish, which is basically "German" with high-pitched voices and mangled consonants. In another they are forced to wear "suicide teeth", large bulky dentures containing poison, making them garble their speech to avoid releasing the poison. In yet another, von Strohm and Gruber are posing as Frenchmen, and are forced to speak French. This comes out as another set of non-words sounding like "Woffel woffel, woffel woffel".
A further episode features a Swedish art dealer inspecting The Fallen Madonna, who pronounces "Heil Hitler!" as "Oil Jesus!"