To change the language or local in Pipedrive, go to Settings > Personal > Profile.
Here you'll find two dropdown menus (among others), 'Locale', and 'Language'
Changing the locale will change the way that things in Pipedrive are formatted - numbers (for example 1,0 for German and 1.0 for English), date format (for example 29/09/2017 for British English, and 09/29/2017 for American English), and other such factors. It will also change the day on which the week starts to reflect that locale or country's standard.
Language, on the other hand, presents a shorter list of options - here you'll find all the languages that Pipedrive is translated into. Change this, and you change the language, but not the format of any of the data.