You can assign keywords to topics by two ways:
1. Enter them into "Keywords" field of the topic editor. Separate individual keywords with commas.
2. Select keyword in the topic editor, right-click it and select "Add to keywords" from the context menu.
Helpinator also has "Auto Index" feature. All you need is to add keywords for each language into "Keywords" section of project tree view. Then Helpinator will append keywords to topics automatically at compile time. To enable "Auto Index" select Project root in the tree, select "General" tab and check "Auto Index" checkbox.