How can we make the fast queries work in multi lingual and Language fall back scenarios? We are using Fast queries across our code to fetch items. and we are facing issue when trying with multi-lingual scenarios fro language fall back. Can someone please help me with this.
If I remember correctly, Fast Query doesn't support language fallback scenarios. I would suggest to change your code and use search indexes to get items.
Sitecore Fast Query does [not] take the context language into account when it checks the value of non-shared fields. The query results therefore include all the items that have one or more language versions that match the query expression.
Source: Using Sitecore Fast Query
While the document is based on Sitecore 6,
fast: query became obsolete with the introduction of Sitecore ContentSearch in versions 7 and above.
You need to update your code to current Best Practices standards. There's good resources available.
fast: query is obsolete. It is also extremely damaging for your runtime environment, as it doesn't scale at all. You are pretty much putting all the load onto your SQL servers running the solution, bypassing all the advanced caching, indexing and so on that Sitecore has to offer.
For additional information on the role of Fast Query and performance, here are a few additional resources.