In order to translate/localise street-fields labels:

  • explode street fields from Stores > Configuration > Sales > OneStepCheckout for either billing or shipping fields 
  • open up i18n/en_US.cs and find the Street* fields . It is suggested to maintain such changes in your theme scope and in your own locale files. 
  • Translate the fields as follows:
"Street Address","Street Address",module,Onestepcheckout_Iosc
"Street Address 1","Street Address 1",module,Onestepcheckout_Iosc
"Street Address 2","Street Address 2",module,Onestepcheckout_Iosc
"Street Address 3","Street Address 3",module,Onestepcheckout_Iosc

can be localised as follows:

"Street Address","Straat",module,Onestepcheckout_Iosc
"Straat 1","Huisnummer",module,Onestepcheckout_Iosc
"Straat 2","Toevoeging",module,Onestepcheckout_Iosc
"Straat 3","Street Address 3",module,Onestepcheckout_Iosc

Trick is that Magento2 uses double localisation for street-field labels on frontend and thus you need to localise any following line by your first localisation. First localised value is not carried over from backend localisation and is reassigned for localisation on frontend based on first value. To overcome this just localise following fields in localised form.