The unit leader or representative should note the personal achievement on the BSA Advancement Report for youth or on the BSA Training Report for adults and submit to the local Council for posting. The strip shown is for speaking German, one of the most common strips.
Most adults earn an interpreter's strip by working or living overseas as part of business.
In most local Councils, there is no formal application which is to be completed. In some local Councils, a special Interpreter Strip application is made available for filing within the unit's records at the local Council. The strips were originally all red, as shown below. However, the older red interpreter strips are getting a little hard to find. The BSA's Supply Division maintains strips on 40 languages, I'm told.