If you wish to change the language or Date/number format in Pipedrive, go to Settings > Personal preferences > Account.
Here you will find the two fields in question — 'Date/number format' and 'Language'

The 'Date/number format' field changes the way that certain fields are formatted in Pipedrive

• numbers (for example: 1,0 for German, 1.0 for English)
• date format (for example: 29/09/2017 for British English, 09/29/2017 for American English)
• week starting date (for example: Europe considers Monday the beginning of the week, while the United States, Canada, and Australia begin on Sunday.)
  
  

The 'Language' field allows you to choose what language is used to display your Pipedrive account. Each user can choose the language they wish for their Pipedrive to be displayed in, without affecting the display language of other users in the Pipedrive account.