Add missing i18n for UI strings

The text of the Arduino IDE user interface has been localized to 12 languages.

Before localization can be accomplished, internationalization must be done in the application's code base:

- Set up infrastructure to export localization data
- Pass all target strings to that infrastructure

While the first of these tasks is completed, the second was not completed for several strings which are part of the user
interface.

Those outstanding strings are hereby internationalized and will be made available for localization.

arduino-ide-extension/src/browser/boards/boards-config.tsx

@@ -306,7 +306,10 @@ export class BoardsConfig extends React.Component<
             type="search"
             value={query}
             className="theia-input"
-            placeholder="SEARCH BOARD"
+            placeholder={nls.localize(
+              'arduino/board/searchBoard',
+              'SEARCH BOARD'
+            )}
             onChange={this.updateBoards}
             ref={this.focusNodeSet}
           />
@@ -344,7 +347,9 @@ export class BoardsConfig extends React.Component<
       });
     }
     return !ports.length ? (
-      <div className="loading noselect">No ports discovered</div>
+      <div className="loading noselect">
+        {nls.localize('arduino/board/noPortsDiscovered', 'No ports discovered')}
+      </div>
     ) : (
       <div className="ports list">
         {ports.map((port) => (

arduino-ide-extension/src/browser/dialogs/user-fields/user-fields-component.tsx

@@ -65,7 +65,11 @@ export const UserFieldsComponent = ({
                     type={field.secret ? 'password' : 'text'}
                     value={field.value}
                     className="theia-input"
-                    placeholder={'Enter ' + field.label}
+                    placeholder={nls.localize(
+                      'arduino/userFields/enterField',
+                      'Enter {0}',
+                      field.label
+                    )}
                     onChange={updateUserField(index)}
                   />
                 </div>