Why, in a history point of view, is Germany called Allemagne/Alemania in latin languages, Germany in English and Deutschland in German

The name Deutschland (German) and other similar names (like Duitsland in Dutch and Tyskland in Danish) is derived from the Old High German word diutisc, which meant 'of the people'. This in turn comes from a Germanic word meaning 'folk' (leading to Old High German diot, Middle High German diet), and was used to make a distinction between the speakers of Germanic languages and the speakers of Celtic or Romance languages. Ultimately, those words come from the Proto-Indo-European word teuta, which meant 'people'. Through the PIE root, Deutsch is related to the Lithuanian word tauta, the Old Irish word tuath and the Old English word þeod, all of which mean 'people' or 'folk.'

The name Germany and other similarly sounding names (like Germania in Italian, An Ghearmáin in Irish and Германия in Russian) is derived from the Latin term Germania, which was first used in the third-century BC. The origin is uncertain, but it's probably that Germania is of Gaulish origin, as it was the Gauls who first referred to the people who crossed east of the Rhine as the Germani, a name which the Romans adopted.

The name Allemagne (French) and other similarly sounding names (like ألمانيا in Arabic, Yr Almaen in Welsh and Alemanha in Portuguese) are derived from the name of the Alemanni. The Alemanni were a loose confederation of Germanic tribes that were also known as the Suebi who lived primarily in what is today the region called Alsace in France and the German Bundesland (Federal State) of Baden-Württemberg. The word Alemanni has it's origins in the Proto-Germanic word Alamanniz, which has two possible meanings, depending on the origin of the prefix Al-. The first possibility is that Al- means 'all,' which would mean that Alamanniz means 'all men.' This suggests that the Alemanni were a diverse confederation of different tribes instead of a unified group. The other possibility is that Al- comes from the Latin term alius, which refers to 'the other.' In that case, Alemanni shares a root with the English word 'alien' and means 'foreign men.'

Finally (and to answer /u/hecklad's question), the name Niemcy (Polish) and other similarly sounding names (like Németország in Hungarian, Німеччина in Ukrainian and نمسا which refers to Austria in Arabic) are derived from the Slavic exonym nemets, which is itself derived from Proto-Slavic nemcy, which means 'mute' but came to refer to 'those who do not speak like us' or more simply 'a foreigner.' This usage is related to the Proto-Slavic autonymical term slovo which means 'speech.' The Proto-Slavic tribes referred to themselves the 'speaking people', as opposed to their Germanic neighbors, who they referred to as 'mutes.' This mirrors the Ancient Greek term βάρβαρος (barbaros) which referred to all non-Greek speakers and roughly means 'those who speak gibberish.'

Sources:

John Joseph Gumperz and Dell Hathaway Hymes, The ethnography of communication. 1972

Mary Fulbrook, A Concise History of Germany. 2004

Kurt F Reinhardt, Germany: 2000 Years. 1961