Hari You can leave translations as blank - Flarum will display original phrase in English if translation is empty. Also, you don't need to translate everything - there is over 200 extensions, but many of them are not that popular. You can focus on completing translation for core + validation + first 50 extensions from popularity ranking and translate rest only on request from users - there is great chance that nobody will need Telugu translation for some less popular extensions.
Secondary language is a language that you know, but you don't want to translate. For example if you had some basic classes of French, you can set French as secondary language and translation in this language will be displayed in translation form - it may help to understand context of this particular phrase, because now you have this phrase in 2 languages inestead of only English.